Summary An ovine placental lactogenic hormone (OPL) was purified from placentas. Principal steps of purification were : saline extraction at pH 9.5, acid precipitation at pH 4.5, ammonium sulfate precipitation, DEAF Sephadex chromatography and gel filtration on Sephadex G75 S.F. This hormone is purified about 1000 fold. It is not- contaminated either by prolactin or by growth hormone. Its activity estimated by radioreceptor assay and biological tests was equivalent to 15 IU/mg of NIH ovine prolactin standard. Apparent molecular weight of OPL was about 20,000–22,000 and its pI was about 7.2 . Isohormones and prohormone phenomenons have been observed. The purified OPL showed no immunological cross-reaction with ovine prolactin, human and ovine growth hormone and human placental lactogen. Mammotrophic and lactogenic activities have been discussed.