All untreated rat sera inhibited hemagglutination of RV and L-S strain of rat virus in high titers. Treatment of rat sera by heat or KIO4 did not remove hemagglutination inhibitors, while kaolin was effective. However, neutralization titers were even lower than H-I titers with kaolin-treated sera. Neutralization tests differentiated between antibodies against the two viral strains. Four strains of rats varied in the percentage of animals with antibodies against rat virus from 0–100%, and these findings are compared with spontaneous tumor rates. Seventy-eight percent of Wistar rats with spontanous adenomata, benign cysts, or X-ray-induced tumors had antibody against one or both viruses, while sera from none of the animals with transplanted tumors exhibited antibody. In Marshall rats, which were shown to be antibody-free, RV caused a lethal infection when injected into newborns within 6–8 days, while the L-S strain caused only 42% deaths in 13–60 days. Injection of older animals resulted in production of antibodies but no signs of disease. None of the animals developed tumors after inoculation with rat viruses. The similarities between the properties and behavior of these viruses to known oncogenic viruses are discussed.