Flow cytometry and the fluorescent dyes DCF and R123 were used to examine oxygen metabolite production in human leukocytes and T-lymphoblastoid Jurkat cells, activated by PMA or by FMLP. When unseparated leukocytes were activated by PMA, oxidative products were generated not only in PMN and monocytes but also to a lower extent in lymphocytes. These responses were correlated with protein kinase C activation. PMA did not, however, induce the synthesis of reactive oxygen species in isolated lymphocytes. FMLP did not affect lymphocyte oxidative metabolism when added to the whole leukocyte mixture, but activated only the phagocyte populations. Similarly, Jurkat cells which alone were unresponsive to PMA, became strongly fluorescent when they were mixed with PMN and treated with this activator. In all cases, they did not respond to FMLP. Superoxide dismutase and catalase addition did not prevent the lymphoid cell response in the presence of phagocytes, whereas Desferal did. These data indicate that under physiological conditions, activated lymphocytes are capable of oxidative metabolism and also evidence some close relation between the leukocyte populations. We discuss the putative mechanism of oxygen metabolite generation in lymphocytes and the role of these metabolites in the immune response.