AbstractThe separation of a fraction corresponding to prostaglandin A1 from yellow onion (Allium cepa) and subsequent purification of that fraction as prostaglandin A1 has led to the identification of prostaglandins in a plant material for the first time. Kilogram quantities of onions were processed and purified by extraction procedures, column chromatography and thin layer chromatography (TLC). Prostaglandin A1 was characterized and identified by a combination of comparative TLC, gas chromatography‐mass spectrographic analysis and blood pressure lowering properties. The results of these experiments are consistent with standard prostaglandin A1. It is concluded that prostaglandin A1 is present in onion.