We studied the effects of natural and recombinant human IL-2 (rIL-2) on secretion of prostacyclin (PGI2), vWf, and tissue-type plasminogen activator (tPA). IL-2 elicited a steady increase in PGI2 synthesis by cultured human umbilical vein endothelial cells (HUVECS) and bovine aortic endothelial cells but had no effect on vWf or tPA. Both purified natural IL-2 (nIL-2) and rIL-2 induced significant PGI2 synthesis. Substitution of the cysteine residue at position 125 of rIL-2 with serine or alanine led to loss of PGI2-stimulatory activity in HUVECS without affecting thymidine incorporation in lymphocytes. HPLC analysis of arachidonate metabolites detected predominantly 6 keto-PGF1 alpha (6KPGF1 alpha) peak. Treatment of cultured endothelial cells with cycloheximide and actinomycin D resulted in inhibition of 6KPGF1 alpha synthesis. The Western blot using a polyclonal antibody against PGH synthase revealed an increment in the 70-kD subunit of PGH synthase by nIL-2 and rIL-2, but not by alanine-substituted rIL-2. We conclude that IL-2 stimulated sustained PGI2 production by a mechanism that includes the de novo synthesis of PGH synthase. This mechanism for regulating AA metabolism probably has important physiologic implications.