Digestion of the pregnancy zone protein with papain at pH 4.5 yields an 18 kDa C-terminal fragment. This fragment consists of the 145 C-terminal amino-acid residues cleaved at Asn- 1 2 8 8 Ile and is homologous to the C-terminal receptor binding fragment of human α 2 -macroglobulin obtained by cleavage with papain. The fragment contains an intrachain disulfide bond between 1 3 0 8 Cys and 1 4 2 3 Cys corresponding to that between 1 3 0 4 Cys and 1 4 1 9 Cys in α 2 -macroglobulin. An oligosaccharide chain, is present in the C-terminal fragment of pregnancy zone protein as in human α 2 -macroglobulin. The PZP C-terminal fragment was demonstrated to bind to the LRP/α 2 M-receptor. Both the pregnancy zone protein and α 2 -macroglobulin fragments bind three mAb's (α1:1, R35, and 7H11D6) generated against α 2 -macroglobulin. The mAb 7H11D6 was generated against the α 2 -macroglobulin-proteinase complex (Isaacs, I.J., Steiner, J.P., Roche, P.A., Pizzo, S.V. and Strickland, D.K. (1988) J. Biol. Chem. 263, 6709-6714) and the binding of this to the C-terminal fragments of both pregnancy zone protein and α 2 -macroglobulin indicates that both proteins use the same receptor recognition site for binding to the LRP/α 2 M-receptor.