The ability of hydrocortisone to modify antigen-mediated inhibition of macrophage migration, an in vitro correlate of cellular immunity in the guinea pig, was investigated. Only the glucocorticoids, hydrocortisone and dexamethasone, significantly blocked migration inhibitory factor (MIF) activity in pharmacologic concentrations. Hydrocortisone had no effect on antigen "processing" by macrophages, nor on the ability of antigen-stimulated peritoneal exudate lymphocytes to produce MIF. Rather, hydrocortisone antagonized directly the inhibitory effect of MIF on the macrophage.