Antibodies to visna virus neutralized the virus in fibroblasts and macrophages but specifically enhanced the binding, penetration, and uncoating of the virus in the latter cells. F(ab')2 fragments of the immune antibody neutralized the virus in fibroblasts but did not enhance the early stages of the virus life cycle in macrophages. Furthermore, these fragments did not neutralize infectivity in macrophages but delayed the appearance of infectious virus in cells after the inoculation of preincubated virus-F(ab')2 complexes.