The coexistence of activated polymorphonuclear leukocytes and lymphocytes in tumor masses and inflammatory tissues suggests the possibility of interaction between secreted neutrophil products and nearby lymphocytes. To test this hypothesis, we examined the effects of neutrophil myeloperoxidase and H2O2 on lymphocytes. Human peripheral blood mononuclear leukocytes were exposed to myeloperoxidase, an H2O2-generating system (glucose + glucose oxidase), and a halide, and were then tested for functional activities. Natural killer activity against K562 cells, lymphocyte proliferation in response to mitogens, and generation of immunoglobulin-secreting cells were all susceptible to oxidative injury by myeloperoxidase and H2O2. The degree as well as the mechanism of suppression was dependent on the glucose oxidase concentration (i.e., the rate of H2O2 delivery). At low H2O2 flux, myeloperoxidase was essential for induction of lymphocyte suppression; as the rate of H2O2 generation increased, suppression became myeloperoxidase-independent and was mediated by H2O2 alone. Various lymphocyte functions were differentially susceptible to oxidative injury by myeloperoxidase and H2O2. The proliferative response to poke-weed mitogen was the least sensitive, whereas antibody formation was the most sensitive. Proliferative responses to concanavalin A and phytohemagglutinin as well as natural killer activity displayed intermediate degrees of susceptibility. In all assays, lymphocyte viability was greater than 90%. Removal of monocytes from mononuclear leukocytes by adherence to glass increased susceptibility of lymphocytes to oxidative injury. Monocytes in proportions within the range present in peripheral blood mononuclear leukocytes protected lymphocyte functions against oxidative injury by myeloperoxidase and H2O2. This study demonstrates a differential susceptibility of various immune functions to oxidative injury by the neutrophil products myeloperoxidase and H2O2, and shows, in addition, that monocytes can modulate these interactions.