The role of lymphocytes in the pathogenesis of fever was investigated by stimulating human blood mononuclear cells in a two-way mixed leukocyte reaction (MLR). After 2-7 d of incubation, MLR supernates contained a factor that was not pyrogenic itself when injected into rabbits; however, these supernates, when incubated with human blood monocytes from a third donor, induced the synthesis of LP. The pyrogen-inducing activity was stable at 56 degrees C, destroyed at 70 degrees C, and was neither dialyzable nor removable by adsorption by anti-human leukocytic pyrogen (LP) attached to Sepharose 4B. Production of this factor was not always correlated with increased thymidine incorporation in the MLR. Its production was absent when peripheral lymphocytes were purified over nylon wool. The concentration of mononuclear cells in the MLR varied from 5 X 10(5) to 5 X 10(6)/ml in round-bottomed tubes. Under the latter conditions, some donor cells produced this factor without stimulation in the MLR culture, but when these cells were cultured on flat-bottomed containers at low cell concentration, autologous production was not observed. These experiments demonstrate the production of a human lymphocyte factor (lymphokine) that induces LP synthesis. This pyrogen-inducing lymphokine may be important in the pathogenesis of fever in certain immunologically mediated diseases.