Six well-characterized specimens of cultured astrocytoma cells were investigated with a panel of macrophage markers. Our results show that the macrophage markers OKM-1(CD11b), OKM5(CD36), EBM11(CD68), HAM56, Factor 13, alpha-1-antichymotrypsin, alpha-1-antitrypsin, ferritin and lysozyme are clearly reactive to neoplastic astrocytes whereas astrocytes in normal brain specimens are not reactive. In order to obtain further confirmation concerning the reactivity of tumor cells in vivo, we simultaneously measured by flow cytometric analysis DNA content and HAM56 immunoreactivity in a freshly obtained tumor specimen. In this experiment we found a marked reactivity of aneuploid cells to HAM56. The macrophage phenotype of malignant astrocytes may reflect a similarity in functions of these cells and tumor-associated macrophages which promote tumor growth via the production of growth factors and angiogenic factors. Furthermore, our findings implicate that demonstration of macrophages within malignant astrocytomas by using macrophage-specific antibodies must be cautiously considered.