Abstract: Blood plasma was found to contain endogenous inhibitor of prostaglandin (PG) synthase (EIPS) and this activity was associated with at least two plasma components, albumin and haptoglobin. Hemoglobin, a stimulant of PG biosynthesis which is bound by haptoglobin, reversed the inhibitory effect of haptoglobin and reduced the inhibition by albumin, acetylsalicylate or indomethacin. It is proposed that in physiological situations the albumin-associated EIPS mainly helps to prevent excessive PG production, whereas in tissue injury the acute phase protein haptoglobin plays a more important role. Possible clinical applications of EIPS are discussed.