The monoclonal antibody MUC 2-63 identifies a glycoprotein antigen with a molecular weight of 32,000 Dalton present on human malignant gliomas but not detected on normal nerve and brain tissues. This was the prerequisite for these clinical studies with 111In-MUC 2-63 for imaging and 90Y-MC 2-63 for treatment. Seven patients with malignant gliomas, 6 with astrocytomas grade III-IV and 1 with a relapsing astrocytoma grade I-11 received the 111In labelled monoclonal antibody MUC 2-63 for in vivo diagnosis. Six patients had an uptake in the tumour of 0.01 to 0.05% of the MUC 2-63- 111In monoclonal antibody. The patient with the relapsing astrocytoma grade I-II had a negative scan due to a radical operation before the diagnostic dose. All patients received treatment with 90Y-MUC 2-63 intravenously in doses ranging from 146 to 830 MBq. Two patients with relapsing grade III-IV astrocytomas demonstrated clinical improvements and CT changes interpreted as necroses. No serious side-effects were observed in the 5 patients who only received one dose. The two patients who received up to 4 doses experienced grade 3 to 4 leukocyte and thrombocyte toxicity, most likely related to the bone-marrow toxicity by 90Yttrium. These data indicate the potential usefulness of the MUC 2-63 monoclonal antibody for in vivo image on humans with brain tumours and for adjunct treatment after operation and external radiotherapy.