Lymphocytes which appeared in the peripheral blood early (approximately 4 weeks) after complete bone marrow aplasia were studied in two groups of patients. Twenty-two allogeneic bone marrow transplant recipients and 12 acute nonlymphocytic leukemia patients (entering remission) were compared to 70 healthy control subjects studied during the same time interval. Studies performed included phenotyping T-cells using monoclonal antibodies and T-cell colony formation in response to phytohemagglutinin. The phenotypic profile for the two patient groups differed from each other and from that of the healthy controls. The total number of circulating cells reactive with OKT4 were significantly depressed in marrow graft recipients while only mildly, but significantly, depressed in leukemic patients. The number of circulating cells reactive with OKT8 were depressed in leukemic patients but were essentially normal in marrow graft recipients. The number of circulating cells reactive with OKla1 and OKT10 were significantly elevated in marrow graft recipients while significantly depressed in leukemic patients. The T4:T8 ratio was significantly depressed for marrow transplant recipients and significantly elevated for leukemic patients. T-cell colony formation in agarose without and with added interleukin-2 was decreased for both groups, more so for marrow graft recipients who virtually lacked the ability to make colonies without exogenous interleukin 2. These phenotypic and functional data suggest that T-cell reconstitution after bone marrow aplasia and profound lymphopenia takes quite different pathways for leukemic patients recovering from remission induction therapy and for recipients of bone marrow transplants. We were unable to correlate T-cell functional response in T-cell colony formation with the phenotypic profile of peripheral blood T-cells.