Direct immunodiffusion comparicon with specific antisera demonstrated that all of the four pregnancy-associated plasma proteins (PAPPs) described in our laboratory are distinct from the pregnancy zone protein (von Schoultz); (α2-pregnoglobulin (Berne); pregnancy-associated α2-glycoprotein (SP3) (Bohn); new serum α2-macroglobulin (Stimson); PAG (Horne); pregnancy-associated α2-globulin (Kasukawa); Pal (McLaren), and Xh protein (Dunston). All the latter proved to be immunologically identical to each other, and apparently represent the same protein described under different names. It was confirmed that none of the PAPPs is immunochemically related to placental alkaline phosphatase. PAPP-A, PAPP-C, and the pregnancy zone protein, but not HPL (PAPP-D), showed a decreased anodic mobility when treated with neuraminidase; their reactivities with antibody were essentially unaffected by the enzyme, however. Certain detergents had no effect on the immunological reactivities of the PAPPs and the pregnancy zone protein in whole plasma, while butanol and desoxycholate partially, and urea, SDS and cetylpyridinium largely inactivated them.