We studied the effect of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) on the cytotoxic action of TNF on MCF-7 human breast cancer cells and on adult bovine aortic endothelial cells. 1,25(OH)2D3 increased the effect of TNF on MCF-7 cells but not on endothelial cells over a wide TNF concentration range. At a suboptimal concentration (1 ng/ml) the potentiation was twofold. The effect of 1,25(OH)2D3 was specific, dose-dependent and apparent at a physiological concentration (0.1 nM) of the hormone. The potentiating effect of 1,25(OH)2D3 on TNF action was abolished by cycloheximide indicating that their interaction requires protein synthesis. Addition of 1,25(OH)2D3 13 h after TNF in a 28-h assay was sufficient to induce its full potentiating effect indicating that the hormone modulates a late event in the cytokine's action. These data suggest that some of the in vivo antitumor effects of 1,25(OH)2D3 may be due to an increase in the anticancer activity of the immune system.