The effect of ketotifen, an antianaphylactic drug which acts in part similarly to sodium cromoglycate, was tested on adriamycin-induced histamine release and toxicity. The intraperitoneal injection of various concentrations of ketotifen induced an important histamine release from rat mast cells. Thirty minutes after the injection, however, no more histamine was present in the peritoneum. When i.p. administered to mice 30 min before adriamycin (15 mg/kg), ketotifen significantly ameliorated the survival time and reduced the cardiotoxicity. On the contrary, when given simultaneously, the antiallergic drug increased the toxic effect of adriamycin.