Cephalothin significantly suppressed in vitro DNA and total protein synthesis in human peripheral blood lymphocytes stimulated by antigens or mitogens. However, similar concentrations of this antibiotic enhanced streptokinase-streptodornase-stimulated production of the lymphokine leukocyte-migration-inhibition factor (LMIF) and directly stimulated production of this lymphokine by otherwise unstimulated lymphocytes from 10 of 12 normal human subjects. Penicillin did not appear to produce these effects. Cephalothin did not interfere directly with neutrophil migration or the interaction of preformed LMIF with neutrophils. Stimulation of LMIF production by cephalothin required viable lymphocytes and was inhibited by puromycin. These results suggest that cephalothin is capable of inducing lymphokine production by human lymphocytes in a manner that appears to be nonspecific in nature. This type of effect could be the basis of some apparently immunologic reactions to this antibiotic.