Mice sensitized intravenously with 300 microgram of BCG cell walls in Drakeol-Tween 80 and challenged intravenously 3 weeks later with 50 mg of old tuberculin released migration inhibitory factor (MIF) into the circulating blood in quantities that could be detected in serum dilutions of 1:64 to 1:128. When thymus-derived lymphocytes (T-cells) were absent at the time of sensitization, as in neonatally thymectomized mice or in athymic nude mice, detectable amounts of MIF were not formed. Sensitized mice treated with either anti-theta serum or anti-bone marrow-derived lymphocyte (B-cell) serum before intravenous challenge with old tuberculin released reduced amounts of MIF into the circulation. Mice lethally irradiated, reconstituted with B-cells, sensitized with BCG cell walls, and then challenged intravenously 3 weeks later with old tuberculin did not release MIF into the circulation. When T-cells were injected at least 10 days before challenge, however, MIF appeared.