A selection experiment, based on a foundation stock of over 700 Herdwick sheep and aimed at producing divergent populations of predictable high susceptibility or resistance to experimental scrapie was begun at this Institute in 1961. Twelve years later, flocks of presumed 100 per cent susceptibility to the Cheviot-dervied scrapie agent SSBP-1 used throughout the experiment, and also manifesting a high incidence of the disease when challenged with other scrapie strains, are used in the Compton scrapie research programme. The highly resistant flocks appear to have a small residual incidence of scrapie cases, often with very long incubation periods. The type of scrapie caused by the SSBP-1 agent is largely under the genetic control of a single pair of autosomal alleles, the dominant allele conferring susceptibility.