Several platelet function tests were performed on 31 patients undergoing major operations, who received prophylactically small subcutaneous doses of heparin. A group of 15 similar patients without heparin served as controls. It was found that postoperatively in both groups (a) the platelet retention in glass bead column was significantly increased (p < 0.001), (b) the platelet aggregation by ADP 1 μM was slightly increased, (c) the collagen-induced aggregation, ADP release and the bleeding time remained unchanged and (d) the platelet counts decreased in the first 2 postoperative days and increased thereafter. There was no difference between patients on heparin and controls.