Endogenous ecotropic viruses isolated from radiation-induced lymphomatous tissue of BALB/c mice have been shown to consist of a collection of variants as assayed by the mobility of virion structural proteins p30, p15, p12 and gp70 on SDS-polyacrylamide gel electrophoresis (SDS-PAGE) (Ellis et al., 1980 a). In this study we show that a similar phenomenon occurs in RF mice, but only with regard to p15 and gp70, and not p30 and p12. Using the distinct and unvarying mobility of the RF viral p12 protein on SDS-PAGE as a marker, we show that the RF-derived ecotropic murine leukemia virus (MuLV) is expressed to the exclusion of the BALB/c-derived ecotropic MuLV in F1 hybrid mice of the BALB/c X RF cross, and that variant viruses expressed in F1 radiation-induced lymphomatous tissue display the pattern of variation characteristic of the RF, and not of the BALB/c, strain.