T cells generated during a second round of ontogeny after autologous bone marrow transplantation (ABMT) represent a unique model of early T- cell ontogeny in an autologous situation. Since grafted bone marrows were pretreated in vitro with the cyclophosphamide derivative ASTA Z 7557, circulating T cells had to be regenerated from reinfused hematopoietic progenitor cells. The T-cell population derived from 25 patients post-ABMT was phenotypically characterized: an increase in CD8+ cells, a low percentage of CD4+ cells, and a median of 12% CD56+ (NKH1+) cells were found. When the T cells were stimulated with phytohemagglutinin (PHA) and phorbol myristate acetate (PMA), defective interleukin-2 (IL-2) secretion was observed. In addition, proliferative responses of the T cells after activation through the antigen-receptor- dependent CD3 pathway, through the CD2 dependent alternative T-cell pathway, and by the lectin PHA were investigated. Despite the presence of CD2, CD3, alpha/beta chains of the T-cell receptor, and CD25+ IL-2 surface receptors, abnormal proliferative responses were obtained even in the presence of exogeneous IL-2. In experiments where the T-cell population was separated into CD4+ cells and CD8+ cells, both the CD4- and CD8+ subsets were unable to respond to activating and proliferating signals. Thus, T cells at early stages of ontogeny not only possess an intrinsic defect in IL-2 synthesis but, in addition, were unable to express functional IL-2 receptors in response to mitogenic stimuli.