HUMAN parathormone1,2, ACTH3,4, insulin and glucagon5 are comparatively easy to determine immunologically because antisera with high activity against the corresponding mammalian hormone preparations also give a definite immune reaction with the human hormones. On the other hand, human growth hormone can only be demonstrated and determined immunologically using aiitiserum against primate growth hormone6–8. Lactogenic hormone (prolactin) has a marked species specificity similar to that of human growth hormone9–11. This communication presents further observations on the species specificity of prolactin.