Lactogen receptors have been identified in human chorion laeve and possibly placenta but not other periplacental tissues. The receptors show the same hormonal specificity, and similar binding parameters (affinity 0.47 × 109 l/m, capacity 175 fmole/mg) to lactogen receptors in mammary gland and liver. The presenceof these receptors suggests they may play a role in mediating anaction(s) ofamniotic fluid prolactin in chorion laeve, such as osmoregulation.