Macrophage migration inhibitory (MIF-like) activity was demonstrated in the supernatant fluids from primary cultures of African green monkey kidney cells infected with simian virus 40 (SV 40) virus. Kidney cell cultures not infected by virus had no MIF activity. Supernatant fluids from continuous cultures of nontransformed and SV 40-transformed human fibroblasts contained MIF-like activity. Productive infection with SV 40 virus results in the production of a lymphokine-like factor, as previously observed in other virus-cell systems, involving mumps virus and Newcast,le disease virus. However, while infection with these paramyxoviruses causes the production of macrophage and neutrophil chemotactic agents as well as an MIF, SV 40 infection does not induce chemotactic factors. The results reported here, taken in conjunction with previous observations by ourselves and others, suggest that the production of lymphokine-like factors (cytokines) may represent a general biologic phenomenon, and that many, if not all, cell types, when appropriately stimulated, may be capable of such activity.