Human adherent peripheral blood leukocytes spontaneously elaborate both a thymocyte proliferative factor and a factor which augments the in vitro anti-sheep erythrocyte (SRC) plaque-forming cell (PFC) response of nu/nu mouse spleen cells. Nonadherent leukocytes do not spontaneously elaborate either factor. The adherent cell-derived factors appear to have an identical molecular weight (approximately 14,500 Daltons) as determined by Sephadex gel filtration. The data support the hypothesis that the molecule(s) mediating both enhancing activities is identical to the previously described adherent leukocyte product, LAF. © 1978.