The effect of concomitant treatment with different antibiotics on the cytotoxicity of epirubicin, bleomycin, estramustine and cisplatin was studied in vitro on fibroblasts (V79) and two cancer cell lines (colon cancer HT29 and lung cancer P31). The cell lines were propagated under standard tissue culture conditions and evaluated as the number of surviving cell clones in comparison to untreated controls. Fifteen commonly used antibiotics were tested and thirteen of these were found to modify the cytotoxic effect in one or several of the combinations tested. One antibiotic agent could affect the toxicity of different cytostatics in opposite directions and there were marked differences between the cell lines tested. Only in one of the situations, the combination of bleomycin and ceftazidim, did the antibiotic cause opposite effects on the toxicity of a cytostatic when comparing fibroblasts and carcinoma cells. A most impressive observation was the pronounced increase in cisplatin-induced cytotoxicity by amphotericin B. In conclusion, the results suggest that antibiotics can interact with the cytotoxicity of antitumoral drugs but that the feature of this interaction is seemingly an erratic phenomenon. Further studies are certainly justified, especially regarding the effects of amphotericin B and its mechanisms in enhancing the cytotoxicity of cisplatin.