Abstract: A novel mitogen-associated protein was isolated from a rat neural retina cell line. The protein copurified with platelet-derived growth factor (PDGF)-A and was therefore termed PDGF-associated protein (PAP). cDNAs corresponding to the protein were characterized from both rat and human cDNA libraries. PAP binds to PDGF with low affinity and enhances the mitogenic effect of PDGF-A but lowers the mitogenic activity of PDGF-B. PAP mRNA is abundant in the brain of newborn rats and is found in several other tissues.