SUMMARYThe effect of polyphloretin phosphate (PPP) on the human cutaneous reaction induced by prostaglandin E1(PGE1) has been studied in thirty subjects. Pre‐treatment of the skin with three intradermal injections of 1oo μg PPP reduced by 37% the magnitude of the erythema induced by intradermal injection of 1 μg PGE1. The PGE1induced weal, which is probably mainly due to PGE1‐induced endogenous histamine release, was not reduced. The specific nature of the PG‐blocking activity of PPP was supported, since PPP did not block the erythema and weal induced by intradermal injection of histamine and bradykinin. Intravenous injection of 100 mg PPP did not reduce the erythema due to intradermal injection of PGE1probably because of too low tissue concentration