Cell-mediated immune responses were studied in Wistar rats by migration inhibition factor (MIF) assays in vitro of lymphoid cells derived from thymus, spleen and peripheral blood and peritoneal exudates and by delayed cutaneous hypersensitivity reactions in vivo. Using PPD and purified diphtheria toxoid as soluble test antigens it appeared that significant migration inhibition was observed only when sensitized thymus and peritoneal exudate cells were used in the direct capillary migration system. Indirect MIF assays, using both capillary and agarose techniques, showed that migration inhibitory activity was found in thymus culture supernatants as well as in spleen culture supernatants of sensitized animals. The MIF activity in thymic cell culture supernatants seemed to be antigen-independent, although in this respect no definitive conclusions can be drawn from the present results. Migration inhibition in direct and indirect techniques always correlated with a positive delayed-type skin reaction. It is suggested that a population of thymus cells of sensitized animals is able to produce MIF on specific antigenic stimulation in vitro. This might support the idea that a cell population within the thymus possesses characteristics of peripheral T cells.