Lymphocytes from guinea pigs with delayed cutaneous reactivity to DNP-OA when challenged with this antigen in vitro elaborated a small molecular weight mediator which is chemotactic for monocytes. The in vitro production of mononuclear leukocyte (MNL) chemotactic factor (CTX) was carrier specific and appeared to correlate with delayed hypersensitivity. Production of this lymphokine by specific antigen or mitogen-stimulated spleen cells occurred within 8–24 h of incubation, preceding measurable lymphocyte proliferation. MNL CTX was found to be sensitive to proteases and resistant to digestion by neuraminidese. Moreover, MNL CTX was distinguishable from C5 and from other guinea pig lymphokines.