Peritoneal lymphocytes obtained from tuberculin sensitive guinea pigs, when cultured with purified protein derivative, elaborate into the medium a substance, presumably a protein, capable of inhibiting the migration of normal macrophages. This migration inhibitory factor appears to possess immunologic specificity, since specific antigen enhances its activity. Sensitized cells upon interaction with antigen continuously produce this factor for up to 4 days. Concomitantly, sensitized lymphocytes undergo those morphologic alterations recognized as “blast-cell” transformation.