A serially progagated cell line (L104) was established by co-cultivation of alung adenocarcinoma (L-1) from a patient with concurrent chronic lymphocytic leukemia and XC, a non-producer rat line, known to carry the Rous sarcoma virus (RSV) genome. Karyotype of the L104 cultures revealed predominantly rat-like patterns; however, about 5% of the cells reacted with HLA antibodies and demonstrated human isozyme patterns. Electron microscopy of L104 cells revealed the presence of C-type particles budding from the cell membranes and in cytoplasmic vacuoles. Virus was not detected in any of the other normal lung, lung tumor or XC cells examined after co-cultivation with XC cells. The particles isolated from tissue culture fluids had the biochemical and biophysical characteristics common to other known mammalian C-type particles and were serologically related to the woolly monkey virus (WMV)/gibbon ape leukemia virus (GaLV) complex. Cross-hybridization between viral 3H-DNA transcripts and cellular RNAs from virus-infected cells clearly show the presence of sequences in the L104 cellular RNA related to both the GaLV/WMV group of viruses and rat viruses. Hydroxyapatite chromatography reveals however that the primate-related sequences in the viral RNA are indistinguishable from WMV in thermal elution profile. The host range of L104 virus appears to vary greatly from WMV in being xenotropic and, in the cell lines thus far tested appears, to infect only rat cells. The virus gave positive KC but negative XC assays. Inoculation of whole cells or cell-free supernatants into weaning hamster did not result in either solid tumors or leukemia. Co-cultivation of appropriate cell lines may represent an approach to the detection of latent viruses in human neoplasia.