The combination of retinoic acid or tumor necrosis factor alpha (TNF-alpha) with interferon gamma (IFN-gamma) resulted in a synergistic amplification of the anti-proliferative effect of IFN-gamma on cultured breast cancer cells. Retinoic acid could be replaced by other biologically active retinoids. This synergism was also observed for the induction of 2'-5'-oligoadenylate-synthetase, an enzyme which is not expressed constitutively on BT-20 human breast cancer cells and not inducible by retinoic acid or TNF-alpha alone. However, both substances augmented the IFN-gamma-mediated expression. On the other hand, only TNF-alpha and not retinoic acid was able to increase the IFN-gamma induced expression of HLA-DR on the cell surface. Both cytokines antagonized the IFN-gamma effect on detachability of cultured BT-20 cells. The combinations of retinoic acid with IFN-gamma increased the down-regulation of specific binding sites for 125I-IFN-gamma. On the other hand, IFN-gamma exerted no effect on the concentration of the cytoplasmic binding protein for retinoic acid. Data obtained in this study demonstrate a different pattern of action between retinoic acid and TNF-alpha regarding their synergism in combination with IFN-gamma.