B y immunochemical methods and simultaneous measurements of several normal plasma proteins, human placenta was shown to contain elevated quantities of four pregnancy-associated plasma proteins (PAPP's). In the order of increasing amounts, PAPP-A, PAPP-C, PAPP-B, and human chorionic somatomammotropin (PAPP-D) all were present in placenta extracts in quantities greater than could be expected on the basis of their content in maternal blood. In sharp contrast, the placental content of pregnancy zone protein could be entirely accounted for by the maternal plasma present in the placenta. All of the PAPP's appeared to be readily extractible from placental tissue with buffered saline, the large bulk of them being solubilized in the first extraction procedure. However, absorption studies indicated that appreciable quantities of the PAPP's were still present in the insoluble placental residue after 12 sequential extractions with saline. The chorioamniotic membranes were not significantly enriched in any of the PAPP's. Immunochemical analysis of unwashed placental tissue extracts for the PAPP's IgA, and IgM (maternal blood derived), as well as albumin and transferrin (maternal and fetal blood derived), permitted calculations to be made of the amount of blood and PAPP's in placenta. On the basis of these data, it was roughly estimated that a 400-g placenta (wet weight) would occupy 312 ml in volume, and would contain 144 ml of blood. Of this blood, 36 ml would be derived from the mother.