Macrophage stimulating protein (MSP) was purified to homogeneity from human blood plasma by selection of biologically active fractions obtained by sequential immunoaffinity and high pressure liquid ion exchange chromatography. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis the molecular mass of MSP was 70 kilodaltons (kD); under reducing conditions two gel bands were seen, at 47 and 22 kD. The disulfide-linked two-chain structure of MSP was confirmed by separation of reduced and alkylated MSP chains. A computer search comparison of six partial sequences of MSP digests showed that MSP has not been recorded in data banks of protein sequences. Two MSP fragments had greater than 80% identity in overlaps of 12-16 residues to sequences in the protein family that includes human prothrombin, plasminogen, and hepatocyte growth factor. The concentration of purified MSP required for half-maximal biological activity was the order of 10(-10) M. In addition to making mouse resident peritoneal macrophages responses to chemoattractants, MSP caused the appearance of long cytoplasmic processes and pinocytic vesicles in freshly plated macrophages. MSP also caused phagocytosis via the C3b receptor, CR1. Whereas resident peritoneal macrophages bind but do not ingest sheep erythrocytes opsonized with IgM anti-Forssman antibody and mouse C3b, addition of MSP caused ingestion. Thus, MSP causes direct or indirect activation of two receptors of the mouse resident peritoneal macrophage, CR1 and the C5a receptor.