Naive CD8 T cells isolated from umbilical cord blood were examined for cytokine production and for surface phenotype before and after priming with anti-CD3 mAb in the presence of selected cytokines. On stimulation with anti-CD3 immobilized on CD32-B7-transfected L cells, naive and primed CD8 T cells release high levels of IFN-gamma but no IL-2, IL-4, or IL-5. IL-12-primed cells, and to a lesser extent IL-2-primed cells, have an increased capacity to produce IFN-gamma but display the same restricted pattern of cytokine production. Cells primed in the presence of IL-4 or IL-4 + IL-2 are capable of producing high levels of IL-5 but no IL-4, and their IFN-gamma-producing capacity is much reduced. Cells primed with both IL-4 + IL-12 produce high levels of IL-5 and IFN-gamma but no IL-4. IL-4-producing CD8 T cells are obtained only if IL-4- or IL-4 + IL-2-primed cells are subjected to a second cycle of activation in the presence of IL-2; restimulation of IL-4 + IL-12-primed cells under identical conditions fails to generate IL-4-producing cells but leads to the development of high IFN-gamma and IL-5 producers. Thus, unlike in CD4 T cells, IL-12 completely inhibits IL-4-induced capacity of CD8 T cells to produce IL-4. However, IL-4 and IL-12 synergize to promote the expression of IL-5. Regardless of the priming conditions, primed CD8 T cells are CD45ROhigh/RA-, CD31high and more than 50% are CD4+/CD8+; activated naive or primed CD8 T cells do not express CD40 ligand.