SUMMARY

The ability of human pituitary growth hormone (HGH) to induce milk formation in the pseudopregnant rabbit and to stimulate the crop gland of the pigeon has been investigated. Assayed against purified sheep prolactin, fourteen preparations of HGH all showed marked lactogenic activity in the rabbit mammary intraductal test and the majority of preparations were highly active. On the other hand, in the pigeon, the classical test aminal for prolactin assay, low activity in stimulating the crop gland was usual and only two preparations showed activity comparable with that shown by puri-fied sheep prolactin. The finding that HGH has consistently high activity in a mammalian lactogenic test, which may in suitable circumstances provide good estimates of potency, is discussed in the light of the possibility that HGH may be the only human pituitary lactogen