As a virological approach to the study of human leukemia, the group-specific (gs) antigens in the human lymphoblastoid cell lines chronically infected with Rauscher leukemia virus, OUMS-11a-R, OUMS-6C1-R1 and OUMS-6C1-R2, were examined by the immunofluorescence technique with anti-gs-1 rat serum and anti-gs-3 goat serum. Immunofluorescence-positive cells with the anti-Rauscher leukemia virus goat serum and anti-gs-1 rat serum were observed in 0.2-0.6% of OUMS-11a-R cells, and in 8-15% of OUMS-6C1-R1 and OUMS-6C1-R2 cells. However, no staining was detected in any cell lines with anti-gs-3 goat serum. The number of fluorescent cells roughly coincided with those of cells producing type-C virus particles as observed by electron microscopic examination. In all instances, the fluorescence was restricted to the cytoplasm. It was diffuse and homogeneous in almost all cases, although granular cytoplasmic fluorescence was sometimes observed. There was no nuclear or perinuclear fluorescence. The specificity of these antisera was confirmed by the occurrence of specific fluorescence in the spleen and liver of mice infected with Rauscher leukemia virus, but not in the control tests of uninfected mice. From these results of gs antigen analyses in the human lymphoblastoid cells chronically infected with Rauscher leukemia virus, it was considered that type-C virus particles propagated in these cells were Rauscher leukemia virus itself used for infection, and it seemed unlikely that the hypothetical viral genome of human origin was activated and human type-C virus particles were rescued.