Spleen cells of C57B1/6J mice immunized with complete Freund's adjuvant produced macrophage migration inhibitory factor (MIF) when incubated in vitro with tuberculin purified protein derivative (PPD). For optimal MIF production spleen cells were cultured for 48 h in a serum-free medium, at a concentration of 2 x 10(7) cells/ml. MIF was assayed in a xenogenic system, using oil-induced guinea pig peritoneal exudate cells as targets. MIF synthesis could also be induced by pulsing spleen cells for 2 h with concanavalin A, phytohemagglutinin, pokeweed mitogen or lipopolysaccharide, followed by culture in plain medium. No MIF secretion was induced by incubation of spleen cells with anti-theta or rabbit anti-mouse IgG sera. Cells producing MIF in response to PPD were characterized as B cells by virtue of being insensitive to anti-theta serum and complement, by being retained on nylon wool, glass bead and anti-Ig colums and by the presence of Fc receptors. PPD-stimulated T cells did not produce MIF. PPD-induced mouse spleen cell MIF demonstrated a moderate loss of activity by heating at 56 and 80 degrees C and was completely inactivated after digestion with chymotrypsin. By fractionation on Sephadex G-200, migration inhibitory activity was recovered in a molecular range of 100,000-12,400 daltons.