Four adults, recently diagnosed, acute leukemic patients received 40 separate platelet transfusion treatments before, during, and after intravenous administration of amphotericin B. Fourteen of these platelet transfusions were administered while the patients also received amphotericin B; 26 platelet transfusions were given before or after amphotericin B therapy. The mean platelet increment for platelet transfusions administered while the patients were untreated with amphotericin B was 58 +/- 6 per cent. While the same patients were being treated with amphotericin B, the mean platelet increment was significantly decreased to 29 +/- 8 per cent (t = 3.1; p less than 0.005). In addition, when the patients were not being given amphotericin B, a highly significant negative relationship (r = -0.77; p less than 0.001) between the values for bleeding time and venous platelet count was present. In contrast, during amphotericin B treatment there was no correlation between the patients' bleeding times and venous platelet counts. We propose that amphotericin B therapy has a direct toxic effect on the function and circulation of transfused platelets. These toxic effects are quickly corrected by discontinuing this antifungal antibiotic.