The A4 and AO10 (110 kd glycoprotein) cell-surface antigens are biochemically distinct markers of cultured human astrocytomas that are expressed by only a limited number of other cultured cell types. To further characterize these two antigens, the authors used immunohistochemical methods to determine their expression in normal human tissues, astrocytomas, and over 100 tumors of other histologic types. They found that A4 is expressed 1) throughout the central (CNS), but not peripheral nervous system (PNS); 2) in smooth muscle and a small number of epithelial tissues; and 3) in reactive glia and in astrocytomas, but not in most tumors of other histologic types. In contrast, the AO10 antigen is expressed 1) in a small subset of CNS neurons, but not in astrocytes, PNS neurons, or other normal tissues; 2) in astrocytomas and reactive glia; and 3) in some additional neuroectodermal tumors, but not melanomas, carcinomas, or sarcomas. These findings show that A4 and AO10 are restricted markers for human astrocytomas in vivo. Furthermore, the antigens show distinct patterns of expression in normal human CNS but appear to be coordinately expressed in astrocytomas and astrocytoma-derived cell lines.