Polyphloretin phosphate (PPP) when infused intravenously into guinea-pigs or cats antagonized the bronchoconstriction and blood pressure changes elicited by prostaglandin F2α and cat paw slow reacting substance (SRS). The dose required to induce a 50% reduction was 40–80 mg/kg PPP. The effects of histamine were not significantly antagonized by the doses used. PPP itself also had a blood pressure lowering effect. The results suggest that it may act as a prostaglandin and SRS antagonist in vivo.