Oral administration of forphenicinol stimulated the production of lipopolysaccharide (LPS)-induced interferon (IFN) in mice up to 21-fold over control when given more than 10 days before the elicitation. The IFN production in mice sensitized with bacillus Calmette-Guerin (BCG) 14 days before LPS injection was further augmented by forphenicinol given after BCG. In addition to the IFN production, forphenicinol increased production of LPS-induced tumor necrosis factor (TNF) in mice sensitized with BCG, and the dose of LPS required to induce TNF was reduced by the drug. Furthermore, forphenicinol augmented the antitumor effect of BCG and LPS on Meth A fibrosarcoma.