A previous paper (Kimberlin & Walker, 1977) described an experimental model of scrapie in hamsters in which the incubation period decreased progressively over the first 4 passages before becoming stable at the 5th and subsequent passes. Studies have been made of some of the agent strains present in brains taken from the 2nd, 3rd, 4th and 6th hamster passes. The results indicate the presence of at least two strains of agent at the 3rd passage level. One of these (431K) is highly pathogenic for mice and the other (263K) has an extremely low pathogenicity for mice. However only one of these strains (263K) is present in hamster brain after the 6th serial passage. It is suggested that the 'adaptation' of scrapie to hamsters may involve the selection, from a mixture, of a single strain which is highly pathogenic for hamsters. The possibility of modification of the properties of agent strains on passage discussed.