Analysis of mouse CD4+ inducer T cells at the clonal level has established that a dichotomy among CD4+ T cell clones exists with regard to types of lymphokines secreted. Mouse T cell clones designated Th1 have been shown to secrete IL-2 and IFN-gamma, whereas T cell clones designated Th2 have been shown to produce IL-4 but not IL-2 or IFN-gamma. To determine if such a dichotomy in the helper inducer T cell subset occurred in man, we examined a panel of human CD4+ helper/inducer T cell clones for patterns of lymphokine secretion and for functional activity. We identified human T cell clones which secrete IL-4 but not IL-2 or IFN-gamma, and which appeared to correspond to murine Th2 clones. In marked contrast to murine IL-2 secreting Th1 clones which do not produce IL-4 or IFN-gamma, we observed that some human T cell clones secrete IL-2, and IFN-gamma as well as IL-4. Southern blot analysis indicated that these multi-lymphokine-secreting clones represented the progeny of a single T cell. IL-4 secretion did not always correlated with enhanced ability to induce Ig synthesis. Although one T cell clone which secreted IL-2, IL-4, and IFN-gamma could efficiently induce Ig synthesis, another expressed potent cytolytic and growth inhibitory activity for B cells, and was ineffective or inhibitory in inducing Ig synthesis. These results indicate that although the equivalent of murine Th2 type cells appears to be present in man, the simple division of T cells into a Th1 and Th2 dichotomy may not hold true for human T cells.