Initial rates of accumulation (Vi) of [3-14C] alpha-aminoisobutyric acid (AIB) by human leukemic leukocytes increased markedly and progressively during 240 minute incubations in amino acid-deficient media. Absolute increments were greater in blast cells from patients with acute lymphoblastic leukemia and with acute myeloblastic leukemia than in lymphocytes from patients with chronic lymphocytic leukemia, but per cent increments did not differ. Time-related increases appear to be restricted to an active mechanism for AIB entry and could not be attributed to cell damage, concomitant alterations in the incubation media, or progressive reduction in transinhibition of AIB entry. Studies with two cell populations revealed these increases to be associated with an augmented Vmax and a reduction in apparent Km, suggesting enhanced capacity and affinity of the transport system for AIB. Time-related increases failed to develop in two chronic lymphocytic leukemia (CLL) lymphocyte populations and were partially reduced in two leukemic blast cell populations during continuous exposure to high extracellular AIB concentrations. Hence, this phenomenon may represent an adaptive response to environmental amino acid deprivation. Adaptation may involve de novo protein and RNA synthesis, since it was not seen in cells which were treated with cycloheximide or actinomycin D. However, unlike previous observations in nonmalignant cells, once triggered, the adaptive response was not completely suppressed in leukemic cells which underwent a large degree of adaptation.