Leukemic leukocytes increase their rates of alpha aminoisobutyric acid (AIB) accumulation when incubated for prolonged periods in amino acid deficient media. The time-dependent increase was prevented by concurrent exposure of cells to cycloheximide or actinomycin D in vitro. In addition, the increase in AIB uptake was not present in leukemic blasts studied in vitro when the cells were obtained from subjects with acute myeloblastic leukemia who had received antileukemic therapy. Cortisol added to cell suspensions in vitro inhibited the development of time-dependent increases in AIB uptake in lymphoid cells, but accentuated the process slightly in myeloblasts. Cortisol administered to a subject with CLL by infusion reduced the time-dependent increase in AIB uptake by CLL cells subsequently studied in vitro. These data indicate that the time-dependent increase in AIB uptake may be a means of testing the sensitivity of leukemic cells to drugs.