The effects of up to 12 daily doses of HPA-23 (ammonium 5-tungsto-2-antimoniate) on scrapie were studied using five experimental models of the disease, some with widely different incubation times. Treatment of animals with HPA-23, starting just before injecting scrapie, produced survivors. In some experiments, animals were fully protected against several hundred infectious units of scrapie. Treatment of animals after infection was far less effective. Prolonging the treatment for several weeks was no more effective than treating for a few days. In a very long incubation model of scrapie, the course of 12 daily doses of HPA-23 represented less than 3 per cent of the total incubation period. HPA-23 may interact with certain cells of the lymphoreticular system, preventing infection and, perhaps, the early replication of agent.