PRB was studied in normal human subjects before and after aspirin ingestion. Aspirin ingestion resulted in a prolongation of individual bleeding times greater than 2.4 min (greater than 2 S.D. beyond the group mean before aspirin) in 62.5% of 48 paired studies. The relationship of platelets retained vs. time was linear during the first 3 min of bleeding before and after aspirin. The mean PRB decreased from 22.1 +/- 9.2 to 9.6 +/- 8.6 (p less than 0.001) after aspirin ingestion. Subjects whose bleeding time was prolonged greater than 2.4 min had a significantly higher mean PRB before aspirin and a significantly greater mean decrease in PRB after aspirin than those whose bleeding time was prolonged less than or equal to 2.4 min. Aspirin ingestion reduced the number of EDTA-irreversible clumped platelets present in wound blood approximately 50% during the second and third minute of bleeding, but large numbers of EDTA-reversible platelet clumps were observed in wound blood before and after aspirin. Although platelet retention was significantly decreased during the first 3 min of bleeding after aspirin, the percent of venous blood platelets present in wound blood just prior to the arrest of hemorrhage was equal before and after aspirin. These observations indicate that aspirin prolongs the bleeding time by decreasing platelet clumping and slowing the rate of platelet thrombus formation in severed blood vessels. The presence of platelet clumps in wound blood after aspirin ingestion indicates that alternative mechanisms of platelet aggregation, independent of the arachidonate pathway of prostaglandin synthesis, proceed in vivo unaltered by aspirin.