Since 1972, telecobalt irradiation plus intrathecal methotrexate (ITMTX) has been successfully replaced in Jena by intrathecal colloidal radioactive gold (198Au) plus ITMTX for meningosis prophylaxis in leukemia. Seventy-three children with acute lymphocytic leukemia (ALL) were given 1.24-4.89 mCi (45.8-181 MBq) of colloidal 198Au IT after successful initiation of remission. During cytostatic therapy, the following relapses occurred: meningosis leucaemica, five patients (6.8%); bone-marrow relapse and the meningosis leucaemica, one patient; and bone-marrow relapse, 20 patients (27.4%). In 18 children, combination chemotherapy was terminated after two and a half or three years of treatment. After that time, one meningeal relapse and six bone-marrow relapses occurred. Within the first 24 hours after application of radioactive gold, headaches, vomiting, and fever occurred in less than 10% of the children. An apathy syndrome, leukecephalopathy, or severe infections, were not observed in a single case. Radioactive gold spreads in the subarachnoid space and is phagocytized by the arachnoidea. The tumoricide effect extends selectively over the space of distribution of the latent meningosis leucaemia. The cerebral parenchyma remains unaffected by radiation. Thus, radioactive gold may be preferable to telecobalt irradiation in preventing central nervous system leukemia.