The cross-reaction between a human placental protein, human and simian placental homogenates and rabbit antiserum to HGH has been explored employing a variety of immunochemical and electrophoretic techniques, including immunoelectrophoresis, double diffusion in agar, passive hemagglutination, hemagglutination inhibition and vertical starch gel electrophoresis. Evidence of an incomplete cross-reaction between these materials and anti-HGH sera was demonstrated. Material with similar immunochemical characteristics was found in pregnancy serum, retroplacental serum and pregnancy urine. An antiserum to the partially purified placental hormone reacted with HGH, MGH and simian placental homogenate. “Native” and purified placental hormones were clearly distinguishable in their physicochemical and immunochemical properties from HGH and MGH. The binding affinity of anti-HGH hemagglutinating antibodies for the placental hormone was less than that for HGH. The same anti-HGH serum used in these studies had been shown previously to localize within the epithelium of the syncytial trophoblast of chorionic villi as early as the 12th week of gestation. Bio-assay in the rat of a partially purified and physicochemically altered preparation of the placental hormone gave evidence of significant growth-promoting activity but of relatively low potency; the slope of the dose-response curve obtained by 2 different types of assay was more flat than for the bovine GH standard. These studies suggest that the human and simian placenta secretes a hormone with growth hormone-like and prolactin-like activity. Although this hormone has been called “placental lactogen” by Josimovich and MacLaren, the ad hoc designation human chorionic “growth hormone-prolactin” is suggested. It is proposed that the placental hormone by virtue of its metabolic effects has an important anabolic function in pregnancy independent of the homeostatic mechanisms regulating HGH and of the momentary demands of the fetus. In addition, it exhibits prolactin activity. Similarities in the relationship of HGH to chorionic “growth hormone-prolactin” and of LH to HCG are discussed.