In a previous study we showed that Visna viruses isolated from leukocytes of long term
infected sheep were antigenically distinct from the virus used in the original inoculum
(Narayan et al. I977a ). In three of the four sheep studied, the neutralizing antibody was
directed exclusively against the inoculum virus, suggesting that the parental virus in the
inoculum had mutated and neutralizing antibody had not been stimulated by the mutant
viruses. In this report we present evidence for continuous mutation of the virus in the
infected animal and describe the biological properties of the mutant viruses and the host
response to them