Incubation of guinea pig lymph node lymphocytes with insoluble concanavalin A (Con A) in serum-free medium yields sizable amounts of migration inhibition (MIF) and mitogenic factors (MF). Various controls confirmed that the activities detected in supernatants from such cultures are not due to free soluble Con A released during culture. Whereas guinea pig MIF does not act upon human and horse monocytes, guinea pig MF stimulated human but not horse lymphocytes. Upon stimulation by insoluble Con A, guinea pig MIF and MF appear to be produced essentially by T lymphocytes. The technique proposed for producing lymphokines appears convenient for further purification and biochemical characterization of culture supernatants.