MOLYBDATE AND HYDROLYSIS OF PHOSPHATES 745 3. It is suggested that the effect is due to three factors. (a) If magnesia mixture is added to the reduced phosphomolybdate complex and the resulting precipitate analysed colorimetrically, recoveries of inorganic phosphate are low. (b) Free molybdic acid is reduced, giving molybdenum blue. Contrary to prevailing opinions, molybdenum blue is extracted from acid solutions by i8obutanol. (c) The specific extinction coefficient is about 4-5 times higher when the colour of the reduced phosphomolybdate complex is developed at 1000 than when it is developed at room temperature. This is true for most reducing agents except stannous chloride, with which a higher extinction is obtained at room temperature. 4. The effect is not observed under the milder experimental conditions of Weil-Malherbe & Green (1952). If it had occurred, a stannous chloride method of phosphate estimation, such as was used by these authors, would have been less subject to error than a method using aminonaphtholsulphonic acid. The author is indebted to Mr R. H. Green for much technical assistance.

Summary1. An apparently pure rabbit iso-antibody marked with 131I has been shown to adsorb specifically to a greater extent on some rabbit cells than on others. This accords with previous evidence that the same iso-antibody exhibits a higher titre with the cells which are shown here to absorb more marked antibody than with those which adsorb less.2. Saturation of two of these red cell types by repeated application of fresh quantities of the 131I iso-antibody until no more is specifically adsorbed, suggests that the number of functional antigenic sites is six or seven times greater on the one than on the other cell.3. There is no apparent difference in the amount of 131I iso-antibody adsorbed by any one type of rabbit cell in comparative titrations using 3·3 % MgSO4.7H2O medium instead of normal saline.