SUMMARY.A method is described for the assessment of the fraction of microcytes from the red blood cell (RBC) size distribution histogram. For quantification of the fraction of microcytes an upper threshold corresponding with a RBC volume of 70 fL is established. The reference interval for the fraction of microcytes covers a range from 0·01 to 0·09.In order to examine the relationship between the fraction of microcytes and the mean cell volume (MCV) values computer simulation studies were performed. The computer simulations are based on a Gaussian distributed reference RBC histogram as generated on a Technicon H 6OOO1H 601 Hematology Analyser. From our studies it is shown that determination of the fraction of microcytes yields a higher sensitivity than MCV values for detecting small populations of microcytes. In particular, measurement of the fraction of microcytes is very sensitive to minor changes in the MCV values if a normocytic RBC population combined with a microcytic RBC fraction shows a wide dispersion in their cell volumes.Examination of a blood smear for abnormalities in red blood cell (RBC) morphology and calculation of RBC indices have been for many years the starting point for the differential diagnosis of anaemia by laboratory methods. The mean cell volume (MCV) value offers an indication for the differentiation of a number of haematological disorders. However, because indices are mean values their usefulness for detecting rather small subpopulations of RBCs is negligible. I For the differential diagnosis of RBC disorders in both symptomatic and asymptomatic patients, as well as for establishing the effect of therapy in an early stage, quantitatively measured characteristics of RBC histograms are valuable.P" One of these quantitative measures is the fraction of microcytes. The fraction of microcytes as derived from RBC histograms will be more objective than information obtained from 680qualitative inspection of blood smears.' Established criteria for the upper discriminating levels of volumes belonging to the fraction of microcytes may very depending on the apparatus used for cell sizing. England stated that an upper limit setting of 56 fL to establish the fraction of microcytes is more sensitive for detecting blood donors with a low transferrin saturation than MCV values.If Fernandez concluded that the fraction of microcytes is useful for sensitive detection of iron deficiency. 9For the Technicon H 6000/H 601 system no information is available dealing with the relationship between MCV values and the fraction of microcytes for apparently healthy individuals or for subjects with haematological abnormalities. In this paper a method is presented for quantification of the fraction of microcytes. By use of mathematical analysis the upper discriminating level for detecting microcytic RBCs is fixed at 70 fL.In order to compare the sensitivity of the fraction of microcytes with that of the MCV values, computer simulated studies were performed on RBC curves consisting of two

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PRELIMINARY NOTES 451tively) are high, and nearly equally so, during the initial intervals. The RNA phosphorus would appear to move as follows: Sp --+ DS --+ RNP. The evidence on the manner in which ribonucleic acid participates in protein synthesis is still fragmentary. If such a participation requires the rapid renewal of the nucleic acid, our observations would suggest that the ribonucleic acid of the RNP particles of the microsomes is not directly involved in the formation of microsomal protein, though the protein precursors, activated in the Sp fraction, may be assembled in the microsomes to form the final polymer. It is perhaps significant that the cytoplasmic supernatant fraction, in which the enzymic activation of amino acids s has been shown to occur ~,7, also contained the ribonucleic acid exhibiting the highest initial uptake of asp.We shall submit more detailed information on these studies in due course. This work, which benefited from the technical assistance of Miss EDITH A. LAWRENCE, has been supported by research grants from the U.S. Public Health Service, the National Science Foundation and the Rockefeller Foundation. The distribution of protease activities on liver cell fractionsUsing hemoglobin as a substrate, DE DUVE and co-workersZ, 2 found the highest cathepsin activity in the light mitochondria fraction of liver homogenates. Earlier MAVER AND GRECO 3 had found that the total mitochondria fraction contains the highest percentage of the total protease activity of liver homogenates not only when hemoglobin, but also when benzoyl-l-argininamide was used as a substrate.As cathepsin is the collective name for the tissue proteinases and other proteases, e.g. dipeptidases and a carboxypeptidase, also occur in the tissues we thought it would be interesting to study the protease activities of the liver cell fractions with synthetic substrates, specific for various proteases. 1:5 rat liver homogenates were prepared in o.25 M sucrose. The homogenate was layered on 0.35 M sucrose and centrifuged for 5-5 rain at 9oo × g in order to sediment the nuclear fraction. The supernatant containing the other subcellular components was centrifuged for 12 min at 18,ooo x g. The total mitochondria sedimented in this way could be separated into the light and the heavy fraction by resuspending them in o.25 M sucrose, centrifuging for 8 rain at ii,ooo × g, and resuspending the slightly pink light mitochondria layer (easily distinguishable from the dark yellow heavy mitochondria layer) by causing a slow rotatory movement in the supernatant by means of a small pestle fitted into the centrifuge tube. The suspension obtained in this way was then transferred to another centrifuge tube and the light mitochondria spun down by centrifuging for 12 min at i8,ooo × g. The whole procedure was carried out at about 2 ° C.The final supernatant obtained by sedimenting the total mitochondria was not fractionated further; thus it still contained the microsomes. In order to avoid damage as much as possible the sedimented fractions were n...