Experimental infection of cattle with bovine immunodeficiency virus (BIV) is characterized by persistent, low levels of virus replication in the absence of clinical disease. A virus neutralization (VN) assay was developed to examine the role of VN antibodies in controlling virus replication in cattle experimentally infected with the BIV(R29) isolate of BIV. All animals developed VN antibody, but there was no correlation between VN titres and restriction of virus replication in vivo. BIV infection did not induce high-titred, cross-neutralizing antibody and there was no evidence for antigenic variation through more than 4 years in vivo. Genetic comparisons among the BIV(R29) inoculum virus and viruses isolated from infected animals identified only limited genetic variation during 4 years in vivo. Moreover, there was no evidence that the observed variation was due to selection. Analyses of genetic diversity in the virus stock used for inoculation indicated a fairly homogeneous population. In the absence of high levels of virus replication and overt clinical disease, there appeared to be little selection of virus variants, resulting in antigenic and genetic stability of BIV(R29) during long-term, persistent infection.