Adoptive transfer of splenic T lymphocytes from DBA/2 mice immunized against Friend erythroleukemia cells (FLC) inhibited the development of visceral metastases and increased the survival time of DBA/2 mice challenged i.v. with parental FLC 24 hr to 2 months later. Immune spleen cells were ineffective in mice pre‐treated with potent neutralizing antibody to mouse IFN α/β (but not to IFN γ), demonstrating the essential participation of endogenous IFN α/β in the inhibitory action of immune T lymphocytes against FLC metastases. These findings suggest that the reported inability of immune T lymphocytes to exert an anti‐FLC effect in immunodeficient DBA/2 mutant beige (bg/bg) mice (unless these mice had also been treated with IFN α/β), may have been due to lower levels of endogenous IFN α/β in DBA/2 bg/bg mice than in normal DBA/2 +/bg mice. Experimental results in support of this hypothesis are presented. © 1995 Wiley‐Liss, Inc.