Five chondroblastomas were examined for the presence of monocyte/macrophage-associated antigens by an alkaline phosphatase-anti-alkaline phosphatase immunohistochemical method. The tumor cell populations were analyzed with eight antibodies reacting with separate antigens on cells of monocyte/macrophage lineage and with antibodies directed against glycoprotein IIIa and factor VIII. The "chondroblastic" tumor cells did not stain with any of the macrophage-associated antibodies. Osteoclast-like giant cells stained with the macrophage-associated antigens EBM11, KB90, leukocyte common antigen, and with the megakaryocyte/platelet-associated antigen C17. Only endothelial cells were reactive with antibody to factor VIII. Our data do not support the postulated histiocytic origin of the neoplastic cells within chondroblastomas; the osteoclast-like giant cells present within these tumors are felt to be both reactive and of monocyte/macrophage lineage.