This work was undertaken to study the heterogeneity of GH in serum and placental and pituitary extracts and to study GH physiology in pregnant women. Two distinct monoclonal antihuman GH (anti-hGH) antibodies (MAb) coded 5B4 and K24 were selected for their high binding affinity and specificity. The 5B4 MAb recognized the epitope comprising the NH2-terminal end of hGH, and the K24 MAb recognized an internal epitope. Both MAbs were used in RIAs to measure serum GH concentrations in various circumstances, including pregnancy. The two RIAs yielded slightly different serum GH results in normal men and nonpregnant women, but the overall correlation between the data was excellent. Since the RIAs were not affected by human placental lactogen, the evolution of serum GH in pregnant women could be studied. In such women, serum GH levels progressively declined to undetectable levels during the second half of pregnancy, while a pregnancy-associated serum GH-like antigen [tentatively called human placental growth hormone (PGH)] appeared in the circulation at midpregnancy and increased thereafter up to term. PGH contained the NH2-terminal epitope of pituitary GH, but lacked the internal one. Consequently, it reacted selectively with the 5B4 MAb only. After delivery, PGH disappeared from maternal serum within 1 h. Amniotic fluid contained low GH concentrations; cord serum contained high GH levels, but no PGH. Thus, PGH appears to be secreted selectively into the maternal compartment. PGH was purified from term placenta extracts. According to its chromatographic behavior, it appears more basic than pituitary 22K and 20K GHs. Size dimorphism was demonstrated; PGH was composed of two entities of 22K and 25K, respectively. Pure PGH, obtained in small quantities by preparative electrophoresis, was found to bind to hepatic GH receptor with an apparent high potency compared to that of pituitary GH, PGH, thus, should act in vivo as a GH agonist sharing most of its biological properties. These results lead to the conclusion that PGH is likely to replace the pituitary hormone in governing maternal metabolism during the second half of pregnancy.