We isolated and characterized 2 strains of type-C retrovirus, R5NX and YACNX, from established lymphoid cell cultures derived from Rauscher-MuLV-induced lymphoma (RBL-5) in a C57BL/6 mouse, and Moloney-MuLV-induced lymphoma (YAC) in an A-strain mouse, respectively. The RSNX and YACNX viruses were compared with the prototype 4070A strain of amphotropic virus isolated from feral mice in California, and were found to belong to the same amphotropic virus class on the basis of viral interference, neutralization tests, and other biological properties. However, they were not quite identical with respect to viral neutralization antigens and restriction enzyme cleavage patterns of the proviral DNA. The possibility that these viruses may have been present in the original tumor together with the R-MuLV or M-MuLV as “exogenous” viruses rather than as contaminants of cell cultures by the 4070A virus is discussed. Although these 1 new isolates were not pathogenic when inoculated into newborn C3H/HE mice, they could establish persistent infections in these mice.