Treatment of FV infected mice with homologous antibody at different stages of viremia resulted in a significant reduction of infectivity titers and of splenomegaly with a concomitant increase in life span. When treatment was begun 4 hours prior to virus infection the animals survived longer than 120 days with no apparent symptoms of the disease. The therapeutic effects achieved in animals treated after virus infection are considered to be due to the ability of the injected antibody to neutralize extracellular virus as it is liberated which thus delays the induction of the Friend disease.