The expression of the B-chain of platelet-derived growth factor (PDGF) was analyzed in 29 human brain tumors (4 astrocytomas, 7 glioblastomas, 3 medulloblastomas, 3 oligodendrogliomas, 7 meningiomas, and others) using monoclonal antibody after digestion with alkaline phosphatase, and compared with proliferative activities measured by in vivo uptake of bromodeoxyuridine. Medulloblastomas contained the highest amounts of PDGF B-chain, some four to eight times more than that in control brain tissue. The most predominant PDGF molecule of the medulloblastoma was 17 kd. Astrocytomas, glioblastomas, oligodendrogliomas, and meningiomas contained predominantly 30 and/or 22-24 kd molecules. Glioblastoma and meningioma proliferative activities correlated closely to PDGF concentrations, with only a few exceptions. Tumors that contained a high level of PDGF B-chain showed high proliferative activity, while tumors with high proliferative activity did not always contain a high level of PDGF B-chain. Tumors that contain many PDGF B-chains may thus indicate malignancy.