To test the hypothesis that prostacyclin (PGI2) is formed via a biochemical interaction between platelets and lymphocytes, we measured eicosanoids by high performance liquid chromatography (HPLC) and radioimmunoassay (RIA). A distinct 6-keto-prostaglandin Fl, (6KPGF1,,) peak was noted when [14Clarachidonic acid ([14CjAA) was added to the mixed cell preparations which was increased by pretreating platelets with 1-benzylimidazole (1-BI). Lymphocytes prelabeled with [14CIAA failed to form 6KPGF1, when stimulated with phytohemagglutinin (PHA) or ionophore A23187. When the prelabeled platelets were suspended together with aspirin-treated lymphocytes and stimulated with ionophore, thrombin, or collagen, a 6KPGF1, peak was detected and enhanced by 1-BI. These results were supported by quantifying the 6KPGF1, content in the HPLCpurified fraction by RIA. Adding prostaglandin H2 (PGH2) directly to lymphocytes led to 6KPGF1,, production. Platelet aggregation and release were inhibited by lymphocytes in a doserelated manner. We conclude that lymphocytes possess PG12 synthase activity which is capable of converting platelet-derived PGH2 into PGI2. PGI2 formed is sufficient to inhibit platelet function.