Single tissue pools of brain and five extraneural tissues were prepared from four clinical cases of bovine spongiform encephalopathy (BSE) and fed to C57B1 and/or CRH mice. The disease was transmitted only to C57B1 mice fed brain. Intracerebral passages at 12 and 18 months of spleen and spinal cord homogenates from this group of mice to C57B1 mice resulted in reduced incubation periods. Similar blind passages from all other groups failed to produce evidence of infection in a cumulative incubation period of 42 months. The results suggest that the probability of human infection occurring as a result of the consumption of non-neural bovine tissues is remote.