It was confirmed that mice bearing many transplantable tumors are infected with a virus-like agent which causes a 5- to 10-fold elevation in the plasma lactic dehydrogenase activity of infected mice without tumors. The agent is non-identical with the polyoma virus and without effect on adult rats and hamsters. Maximum titers of 109to 1010ID50per ml of plasma were observed within 36 hours after infection. Subsequently the titer decreased to 105to 107ID50per ml and remained constant thereafter. The plasma lactic dehydrogenase reached maximum activity about 96 hours after infection and remained elevated indefinitely. The virus was present in feces and in a variety of tissues and organs in relatively high concentrations. Liver and spleen yielded the highest titers.Five electrophoretically distinct forms (isozymes) of lactic dehydrogenase were separated from mouse tissues. Infection of mice resulted in an increase of the slowest migrating isozyme in the plasma. Liver, spleen, and erythrocytes were each found to contain only this isozyme while other tissues and organs contained mixtures of lactic dehydrogenase isozymes. The plasma of tumor-bearing mice contained more of the slowest migrating isozyme than infected mice without tumors.