A simple procedure is described for the purification of pregnant mare serum gonadotropin (PMSG). The purification of PMSG from serum employed pH fractionation with metaphosphoric acid, alcohol precipitation, gel-filtration on Sephadex G-100, and chromatography on sulfoethyl-Sephadex C-50. The specific activity of the final product averaged 15, 800 IU/mg and was obtained in yields of 50-80% of the activity initially present in un-fractionated serum. The preparation obtained by this procedure has been characterized with respect to biological activity, electrophoresis on columns of polyacrylamide, and ultracentrifugation.