A lymphokine produced by antigen stimulated lymphocytes, induces blastogenesis in cultures of lymphocytes which are not sensitive to the specific antigen. The in vitro production of this factor (MF) was accomplished utilizing peritoneal exudate (PE) cells from Coccidioides immitis infected guinea pigs. Production of MF by lymphoid cultures paralleled skin test reactivity of the donor animal. Removal of adherent cells from the PE population did not decrease the production of MF; conversely, a more significant production of MF was effected by the adherent cell depleted populations. Maximal production of MF was achieved at non-adherent cell concentrations from 4 X 10(6) to 8 X 10(6) cells/ml. Cell concentrations below 4 X 10(6)/ml produced material which inhibited DNA synthesis in test cultures. MF was separated from the inhibitory substance(s) by column chromatography of the crude preparations on Sephadex G-75. Inhibitor(s) eluted in the void volume (VO), and the MF eluted in an effluent volume (Ve) which was greater than the total bed volume (Vt) suggesting that MF is adsorbed by Sephadex beads.