The alpha granule content of platelets, as indicated by the amount of beta-thromboglobulin (beta-TG) in lysed platelet rich plasma was measured to determine whether platelet stimulation resulted in a circulating population of granule-depleted platelets. In 101 normal controls, with a mean platelet count of 242.6 +/- 6.5 X 10(9)/1, the mean platelet beta-TG content was 55.9 +/- 1.2 ng/10(6) platelets. There was a significant reduction in both these parameters (mean platelet count 195.5 +/- 5.8 X 10(9)/1 (P less than 0.001), mean platelet beta-TG 50.0 +/- 1.2 ng/10(6) platelets (P less than 0.01)) in 74 patients with prosthetic cardiac valves. In 24 patients, cardiopulmonary bypass surgery caused a much greater reduction in median platelet count from 210 X 10(9)/1 to 11.1 X 10(9)/1, two hours after surgery (P less than 0.001) but no overall change in platelet beta-TG. However, five patients who experienced diffuse haemorrhage in the postoperative period had a lower median platelet beta-TG (35.5 ng/10(6) platelets) than the other 19 patients (51.0 ng/10(6) platelets) (P less than 0.05).