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THE introduction of isotope techques and the availability of a suitable isotope label have greatly facilitated the study of the life-span and the sites of destruction of erythrocytes in haemolytic anaemia. Radioactive chromium (51Cr) has several advantages which make it especially suitable for this purpose : the patient's erythrocytes can be labelled and re-introduced into his own circulation, and so studied in their natural environment; chromium liberated by erythrocyte destruction is not uthzed to relabel other erythrocytes, and the specific activity of the radloactive chromium is sufficiently high for the labelling to be attained with a small and apparently non-toxic amount of chromium. Furthermore, in addition to determining the intensity of haemolysis by measuring the rate of elimination of erythrocytes from the circulation, it is possible to learn somethmg of the nature of the haemolytic process by studying the pattern of elimination. The effect of treatment on the survival of a patient's erythrocytes can also be studied and the sites of their destruction determined by counting in vim, with special reference to the role of the spleen and liver. Jandl, Greenberg, Yonemoto and Castle (1956), Hughes Jones and Szur (1957) and others have shown that the in vivo counting t e c h q u e can be of value, too, in predicting the response to splenectomy.In the present paper we describe the results of survival and in vivo counting studies, using 51Cr,.[carried out on patients suffering from hereditary spherocytosis, hereditary elliptocytosis, hereditary non-spherocytic haemolytic anaemia, sickle-cell disease, paroxysmal nocturnal haemoglobinuria and auto-immune acquired haemolytic anaemia. METHODS Erythrocyte SurvivalThe patient's own erythrocytes were labelled with 51Cr by the method of Mollison and Veall (1955). In a few instances the erythrocytes of an apparently compatible donor were labelled and introduced into the patient's circulation.The erythrocytes from I 5 to 2 0 nil. of blood .collected into sterile acid-citratc-dextrose (ACD) anticoagulant were labelled with approximately IOO uc of NaZ5lCrO4 having a specific activity of 10 PC per ug. After washing with saline, the labelled cells were suspended in s a h e and reinjected. Blood samples were obtained after 10 and 15 minutes, and thereafter at intervals of 1-4 days. The radioactivity of the blood was measured in a well-type scintillation counter with a thallium-activated sodium iodide crystal of 5 cm. diameter. A counting rate of 105 counts per min. per uc was obtained, so that with the dose of 51Cr administcred a count double the background was still present when the blood contained only 3 per cent of its original radioactivity. The measurements of the blood samples obtained 10-1 5 minutes after the injection of the labelled cells were used as a standard for comparison with all subsequent measurements.The measurements were corrected for radioactive decay by reference to a semipermanent agar standard. Correction for elution of chromium from the erythrocytes wa...

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