Studies carried out over the past 4 decades on factors controlling the susceptibility to viral leukemias in mice have shown the existence of specific controlling gene loci. Studies using various mouse strains and different leukemia viruses provided evidence for the fact that the controlling genes may act either directly or indirectly. In vitro experiments have shown controlling effects directed by the prospective target cells themselves, in which effective virus infection and/or replication was inhibited. From experiments in vivo it became apparent that susceptibility may be controlled through indirect means, mediated by immune relations and other factors involved in the basic regulations of hemopoiesis. The present paper presents a brief review of the various genes known to control susceptibility and resistance to murine leukemia viruses. Certain conflicting or contradictory statements in the literature relating to the nomenclature of particular genes are discussed and some minor revisions in this field are proposed.