Calusterone, 200 mg daily by mouth, significantly raised platelet counts of 80 women with advanced breast cancer. It showed no erythropoietic activity. Simultaneous measurements in 27 patients receiving Δ1-testoloactone, 1000 mg a day, were not significantly different from those of 21 untreated patients. Calusterone is clinically well tolerated, is weakly androgenic, and, like other 17-alpha alkylated steroids, causes bromsulphalein retention but no hepatocellular damage or jaundice. Its thrombocytopoietic effect in women with advanced breast cancer warrants investigation in patients with thrombocytopenic purpura or aplastic anemia and in patients undergoing chemotherapy.