1. When whole blood was incubated in vitro with S-35 L-cystine and L-methionine, the blood cells became radioactive.
2. Preincubation of whole blood from normals and from patients susceptible to agranulocytosis with chlorpromazine showed no effect upon uptake of S-35 L-cystine and L-methionine by leukocytes.
3. The in vivo administration of S-35 L-cystine was followed by the appearance of radioactive leukocytes. Peak radioactivity occurred in leukocytes in 5 to 12 days.
4. Pretreatment of test subjects with large doses of chlorpromazine did not block the uptake of S-35 L-cystine by leukocytes in vivo. Leukocytes of women showed an increase in the incorporation of S-35 L-cystine, in vivo. Studies performed in vivo on two persons during recovery from agranulocytosis showed enhanced uptake of L-cystine in one and a normal uptake in the other.