TXB2 formation in PRP after thrombin or arachidonic acid stimulation was determined in 23 young compensated insulin-dependent diabetic patients, and in 10 control subjects of equivalent age. No significant difference in TXB2 synthesis during the times sampled was observed. Six out of 23 diabetics had circulating immune complexes (CIC) in their sera as detected by the C1q-SP; after the addition of arachidonic acid and thrombin the amount of TXB2 in PRP from CIC-positive patients was significantly higher than in PRP from CIC-negative patients. The increased synthesis of PG endoperoxides in CIC-positive patients could be a direct effect of antigen-antibody complexes on platelets.