A monoclonal antibody (UJ181.4) was labelled with131I and given intrathecally to a patient who was critically ill with neoplastic meningitis due to a disseminated pineoblastoma. The target antigen had first been demonstrated on tumour cells by immunocytological testing of cerebrospinal fluid (CSF). As a preparation for therapy, a test dose of13II-UJ181.4 antibody was given simultaneously with a control antibody labelled with125I. Good evidence ofin vivoimmunolocalisation was obtained by external gamma counting and by analysis of the radiolabels on centrifuged CSF cells. Specificity ratios of between 10:1 and 20:1 were obtained by these methods. A single relatively small therapeutic dose of131-I (870 MBq) given by the intrathecal route, resulted in a marked clinical improvement and sustained remission for 22 months.