Summary: Incubation of lactic dehydrogenase virus (LDV) with mouse anti-LDV resulted in the formation of an infectious virus-antibody complex (sensitization). Virus sensitization preceded neutralization and the more highly sensitized the virus became the more resistant it was to neutralization. Sensitized virus, in contrast to unsensitized virus was readily neutralized by goat antimouse serum or goat anti-mouse γ-globulin. Neutralization of sensitized virus by anti-γ-globulin occurred within 1 min at 4°C. The amount of virus neutralized was related to the degree of sensitization, the concentration of anti-γ-globulin and free γ-globulin in the reaction mixture and the specificity of the anti-γ-globulin. Anti-γF, anti-γG and anti-γA neutralized sensitized virus, whereas anti-γM had no effect. The Fab fragments of papain-digested goat anti-mouse serum partially neutralized the sensitized virus and blocked further neutralization by undigested anti-mouse serum. The Fc fragment had no effect. These findings point to the potential usefulness of sensitized virus as a tool for detecting and studying various immunoglobulins.