Abstract: A high molecular weight extract of mammalian blood plasma and serum proteins of the group Cohn fraction IV, Cohn subfraction IV.sub.1, IV.sub.4 and haptoglobin, have been found to contain activity which inhibits endogenous prostaglandin synthetase. This extract is therefore of utility in treating or alleviating pathological conditions associated with excessive or unbalanced snythesis of endogenous prostaglandins, for example, in reducing bronchoconstriction or treating arthritis. For this purpose, the extract may be administered by injection, by inhalation, or topically.