Sequential intraperitoneal administration of a low molecular weight enhancing factor, isolated from egg fluids, in mice infected with Rauscher leukemia virus (RLV) significantly stimulated viral replication. This was evidenced by elevated levels of viral DNA polymerase in mouse sera. This stimulation of viral replication correlated with aggravation of viral leukemogenicity, as reflected by increment of splenomegalic response and shortening of survival time. The in vivo potentiation of replication and leukemogenicity of RLV is achieved at least partly by the enhancing factor at the cellular level, since RLV replication was found to be stimulated also in an in vitro system. It is assumed that the stimulatory effect of the enhancing factor on viral replication is connected with its stimulatory effect on the synthesis of cellular DNA.