198Au radiogold-colloid (Behring) was injected as "prophylactic" therapy against central nervous system leukaemia in 26 children with acute leukaemia (22 acute lymphatic and 4 myeloid). During the period of observation (1.3. 1972 to 1.9.1975), 23 children remained free of central nervous system recurrences: 16 children are alive 3 to 39 1/2 months later, seven died after 4 1/2 to 29 1/2 months from the underlying disease. In one child CNS leukaemia developed after about five months, in another one during a second haematological recurrence, while the third one developed retrobulbar infiltration in the optic nerve. In general, 2 mCi 198Au gold-colloid was injected intrathecally via a lumbar puncture. The method was well tolerated.