Prolactin was purified from bullfrog adenohypophyses by extraction of acetonedried powder with acid acetone and chromatography on DEAE-cellulose and Sephadex G100. The bullfrog prolactin had a molecular weight of 23, 000 as determined by SDS gel electrophoresis. The isoelectric point was 5.76 when determined by electrofocussing. The amino acid composition of the purified prolactin closely resembles that of mammalian prolactin. The prolactin was found to be 3.6 times as potent as bovine prolactin in promoting collagen synthesis in the tadpole tail fin.