125I-labelled human pregnancy zone protein complexed with chymotrypsin was removed from the circulation with a half-time of 2.3 min after intravenous injection in rats. After 6 min about 67% of the label was present in the liver and about 3% was in the spleen, both in male and in female pregnant rats. The half-time of removal was more than 30 min for native pregnancy zone protein. Uptake into other organs, including placentae and feti, was negligible. 30 pM labelled pregnancy zone protein X chymotrypsin was specifically bound to rat hepatocytes and adipocytes and to human fibroblasts and monocyte-derived macrophages at 4 degrees C. Binding was almost completely abolished by a saturating concentration of unlabelled alpha 2-macroglobulin X trypsin. Binding of 15 pM labelled macroglobulin complex was completely abolished by a saturating concentration of pregnancy zone protein X chymotrypsin. In rat hepatocytes, binding of pregnancy zone protein complex was lower than that of alpha 2-macroglobulin complex at low ligand concentrations. Half-maximal receptor occupancy was obtained with about 300 pM pregnancy zone protein complex. Unlabelled alpha 2-macroglobulin or pregnancy zone protein complex failed to accelerate dissociation of the labelled pregnancy zone protein complex under conditions where dissociation of alpha 2-macroglobulin was markedly enhanced. It is concluded that pregnancy zone protein and alpha 2-macroglobulin complexes bind to the same receptors. The quantitative differences may be related to the fact that alpha 2-macroglobulin is a tetramer whereas the functional unit of pregnancy zone protein is probably a dimer.