A new method, based on the observation that leukocytes contain PGI2-synthetase activity, was used to measure the effect of pharmacological levels of sulphinpyrazone on the TXA2 and PGI2 production in whole blood. Four human volunteers took 400 mg sulphinpyrazone twice daily for 5 1/2 days. Blood was drawn before the study started, 3-4 h after the initial dosage, 12 h after the 10th dosage on the 5th day, and 3-4 h after the 11th (final) dosage on the 6th day. Collagen was added to the citrated blood samples to stimulate prostaglandin production. Aliquots were removed at regular intervals and TXB2 and 6-keto-PGF1 alpha measured by radioimmunoassay. The production of PGI2 was significantly inhibited in all the samples collected after sulphinpyrazone intake (p less than 0.01). The production of TXA2 was significantly inhibited only in the samples collected 3-4 h after sulphinpyrazone intake on the 6th day (p less than 0.01). These results confirmed the cyclooxygenase inhibitory action of sulphinpyrazone and also showed that in the whole blood system, PGI2 production was more effectively inhibited than TXA2 production.