The cytotoxic activities of 2 novel distamycin-A derivatives, FCE 24517 and FCE 25450A, alone and in combination with tumor-necrosis factor-alpha (TNF), were studied. Both drugs, especially FCE 25450A, analyzed extensively here, inhibited the growth of HL60 promyelocytic cells, and human SV80 and murine L929 transformed fibroblasts in a dose-dependent manner. The growth-inhibitory potential of sequential exposure to the distamycin-A analogs and TNF was determined. A 4-hr treatment of L929 fibroblasts with 100-1,000 ng/ml FCE 25450A, followed by 2 ng/ml TNF, resulted in a synergistic anti-proliferative effect. The synergism of FCE 24517 with TNF was less profound. Experiments to elucidate the mechanism underlying the cooperation revealed that FCE 25450A pre-treatment almost completely abolished the elevated tyrosine phosphorylation of a 137-kDa and other membranal proteins and prevented the de-phosphorylation of another protein band observed in L929 cells in the presence of TNF. FCE 25450A alone induced no changes in the phosphotyrosine profile of the cells. The effect of FCE 25450A was counteracted by the tyrosine-phosphatase inhibitor orthovanadate. In parallel, the inhibitor also diminished the antiproliferative action of the FCE 25450A/TNF combination. These findings suggest that, beyond their cytotoxic effects as single agents, the distamycin derivatives increase the sensitivity of cells to TNF. This effect is governed via the inhibition of TNF-induced tyrosine phosphorylation of specific proteins which are probably involved in the development of TNF resistance. Thus, protein de-phosphorylation might provide an additional mechanism of action of these novel distamycin-A-derived drugs.