Visna virus is a lentivirus which causes fusion of infected cells in vitro. Two types of fusion occur. Fusion from without requires no viral replication and a relatively high multiplicity of infection; fusion from within results from the replication of virus in cells. By using fusion from without as an assay, the mechanism of fusion by visna virus was investigated. Immune sera which contained both anti-fusion and neutralizing antibodies interacted with the virus with rapid kinetics in blocking fusion but relatively slow kinetics in the virus neutralization assay. By using visna virus and an antigenic variant, the epitopes responsible for fusion and virus neutralization were shown to be different. Antigenic variation of visna virus resulted in alteration of the neutralization epitope and conservation of the fusion epitope. This suggested that there were two populations of antibodies and that the viral epitopes for fusion and neutralization were separate. These data suggest that visna virus is capable of infecting cells via two pathways: one via the fusion site and the other via the viral epitope which mediates neutralization.