Ribavirin, when administered intraperitoneally to mice infected with Friend leukemia virus, significantly inhibited development of the disease as determined by inhibition of virus-induced splenomegaly and viable virus titers in the spleen and plasma of the infected animals. The drug was effective whether administered ip in multiple daily treatments, treatments every three days, or single injection. Greatest efficacy was seen when therapy began early in the infection, presumably while the virus was in its eclipse phase.