T cells recovering after bone marrow transplantation (BMT) were analyzed for their phenotypic and functional features by two-color immunofluorescence and a high efficiency cloning technique. A predominance of cells co-expressing natural killer (NK)-related surface antigens, such as Leu 7 (CD57) and CD11b, was detected within both the CD4+ and CD8+ subsets from 5 months postgrafting onward. Such cells are virtually absent among normal circulating CD4+ cells and account for a minority (approximately 30%) of normal CD8+ cells. Postgrafting T cells representative of the whole range of NK-related antigen co-expression were selected from six patients for clonal analyses. In control subjects, 63% and 41% of the CD4+ and CD8+ clones, respectively, produced interleukin-2 (IL-2) whereas approximately 30% of either CD4+ or CD8+ control clones produced interferon (IFN)-gamma. At variance, and irrespective of their CD4+/CD8+ phenotype, lower proportions of BMT recipient-derived clones produced IL-2 (20% and 12%, respectively), whereas the majority of both CD4+ and CD8+ clones (75% and 71%, respectively) released high amounts of IFN-gamma. Purified populations of CD57+/CD11b+ v negative cells from two BMT recipients and two control subjects were cloned and subsequently evaluated for IL-2 and IFN-gamma production. CD57+/CD11b+ cell-derived clones were poor IL-2 producers in both normal subjects and BMT patients. In contrast, IL-2- producing clones were frequent (62% to 79%) among those derived from CD57-/CD11b- cells from normal subjects, whereas they were still represented at lower than normal proportions, ie, 25% to 41%, among clones generated from BMT recipients. CD57+/CD11b+ cells gave rise to comparably high proportions of IFN-gamma producing clones in both normal subjects and BMT recipients (approximately 80%). In contrast, IFN-gamma producing clones were approximately 25% to 50% of CD57-/CD11b- cell-derived clones in both normal subjects and BMT patients. Therefore, while the predominance of NK-related antigen-positive T cells may be predictive of poor IL-2 and high IFN-gamma production, the immune derangement in long-term BMT recipients is further enhanced by the finding that all T cells may be poor IL-2 producers. It is also suggested that IL-2 production is a preferential function of T cells that do not express CD57 and CD11b, whereas IFN-gamma production is attributable to T cells that express CD57 and CD11b.