The pathologic consequences of infection of newborn mice and rats with MuLV (Scripps leukemia virus--SLV) were observed. Serum MuLV p30 concentrations of most strains were elevated 20 to 100 times that of controls while MuLV gp70 levels were elevated only 1.1 to 14.8 times, probably reflecting in part the higher concentrations of gp70 in control sera but also indicating the lack of parallelism in regulation of synthesis of these two viral antigens. Infected mice of most strains developed immunologic diseases, including antinuclear antibody and glomerulonephritis, but not Coombs' antibodies. The nature and severity of the immunologic disease varied considerably, depending upon the genetic character of the host. Most infected animals developed lymphatic leukemias, but four strains showed partial to complete resistance to SLV oncogenesis: BALB/c (nude); C57 Bl/6; (NZB times NZW) F1, and (NZW times BALB/c) F1.