ALMOST 30 years after the initial discovery of the prostaglandins, Bergstrom et al., succeeded in isolating and identifying prostaglandins (PGs) E1, E2, F1α and F2α from sources including human semen1. It was subsequently claimed2 that the 19-hydroxy derivatives of prostaglandins A and B were also present. These compounds were later found at an average concentration of 40 µg ml−1 (ref. 3), In a detailed study of semen PGs we have used a new approach for the measurement of the E prostaglandins, which involves protecting the unstable β-ketol system by oximation in the unextracted semen, thus ensuring minimal degradation to the A or B series. Using this technique we found that the levels of 19-hydroxy derivatives of PGA and PGB are relatively low whereas two new PGs, later identified as 11α,15,19-trihydroxy-9-keto prost-13-enoic acid (19-hydroxy PGE1) and 11α,15,19-trihydroxy-9-keto prosta-5,13-dienoic acid (19-hydroxy PGE2) were present at an average total concentration of 100 µg ml−1. These compounds can be isolated together as the methyl ester/methyl oxime derivatives by thin layer chromatography and the individual components can be separated by gas chromatography as the methyl ester/methyl oxime/tert-butyl dimethyl silyl ethers4.