Chorionic gonadotrophin (hCG) has been known for 58 years. Nearly the only reason for measuring it in blood or urine is as a pregnancy test. Placental lactogen (hPL) was discovered 24 years ago, but to all intents and purposes the only clinical application for hPL assays is as an index of placental function. The first of a new series of placental proteins, Schwangerschafts protein I (SP 1) was announced 14 years ago (Bohn, I97I ). It has fared no better, and for the same reason: we don't know what its function is. Not only is the function of SP 1 unknown but even its structure is open to question. It was originally shown to be a single peptide chain with a molecular weight of 9o ooo and a /31 electrophoretic mobility (Bohn, i972). Now it appears that the blood of pregnant women also contains a much larger protein, 43o ooo daltons, with an ~2 electrophoretic mobility but having antigenic determinants in common with the protein originally described by Bohn (Teisner et al, i978). It is probable that the second protein consists of SP 1 in combination with another serum protein (Ahmed and Klopper, 198i ). Bohn's original protein is now more accurately called SP1// and the new protein SPI~. In this review the designation SP 1 will be retained, using it to apply to the mixture of SP1/3 and SPI~. In pregnancy serum more than 9o per cent of the mixture consists of SP1/~ and the small proportion of SPitz binds more weakly to antisera (Ahmed and Klopper, I982 ). To all intents and purposes what is measured is SP~//. One fact about SP 1 is securely established. It is a product of the trophoblast, secreted into the maternal circulation in increasing measure as pregnancy advances. The evidence for its trophoblastic origin is sound. Immunofluorescent studies have shown it to be present in the syncytiotrophoblast of man and monkey (Sedlacek, Rehkopf and Bohn, 1976 ). Cultures of placental cells produce SP 1 (Chou, Rosen and Mano, I98I), and placental explants incorporated labelled amino acids into immunoreactive SP 1 (Horne et al, i976 ).