Summary We developed an IgGI mouse monoclonal antibody (ONS-M21) directed against a cell surface antigen of medulloblastomas and gliomas in immunisation of mice with the ONS-76 medulloblastoma cell line. The antibody specifically reacted with medulloblastomas, supratentorial primitive neuroectodermal tumours (SPNETs) and gliomas, but not with other neuroectodermally derived tumours (neuroblastoma and melanoma) or with other kinds of tumours (meningioma, neurinoma, leukaemia, and small cell lung cancer). No reactivity was identified with normal body tissues, including peripheral blood cells. Characterisation of the ONS-M21 antigen showed that it was a trypsin-sensitive glycoprotein with a molecular weight of 80 kDa on SDS-PAGE. The pattern of reactivity and the biochemical properties of this antigen were different from those of other markers of medulloblastoma. These results indicate that ONS-M21 detects a new tumour-associated cell surface antigen specifically expressed by medulloblastomas, SPNETs, and gliomas. This is the first report that medulloblastomas may share common cell surface antigens with gliomas, although most studies have concluded that medulloblastoma has a predominantly neuronal phenotype. The lack of reactivity with normal tissue implies that ONS-M21 has potential applications as both a diagnostic tool and a therapeutic agent.Medulloblastoma is the most common primitive neuroectodermal tumour (PNET) of the central nervous system in children (Rorke, 1983;Rorke et al., 1985;Dehner, 1986). The survival rate of patients with medulloblastoma has increased over the past four decades due to multiple factors, including improvement of surgical techniques and postoperative care, but mainly due to advances in whole-neuroaxis radiotherapy (Farwell et al., 1984;Hershatter et al., 1986). However, despite the recent advances in the treatment of many other childhood malignancies by combination chemotherapy, the long-term prognosis of medulloblastoma is still poor. The major reason for treatment failure is that recurrent tumours become insensitive to radiotherapy and chemotherapy (Pastan & Gottesman, 1987). In this situation, a potential new therapeutic approach is the utilisation of monoclonal antibodies, as is done for autologous bone marrow transplantation with purged bone marrow (Coombes et al., 1986).So far, ten monoclonal antibodies (mAbs) directed against medulloblastomas have been described Allan et al., 1983;Jones et al., 1984;Gross et al., 1986;Wikstrand et al., 1986;Gibson & Kemshead, 1987;Feickert et al., 1989;Jennings et al., 1989;Takahashi et al., 1990), but most of these antibodies also show reactivity with a broad range of other tumours and with normal tissues, making their in vivo use problematic.In the present study, we tried to develop mAbs with an increased specificity for medulloblastoma. Assuming that any mAb will have some degree of cross-reactivity with normal tissues, we selected those which at least did not react with peripheral blood cells or normal brain tissue, thus increasing the po...