Correlation was close between in vitro complement fixation, immunodiffusion, and passive cutaneous anaphylaxis tests with the S-63 and GC murine leukemia viruses and immunofluorescence reactions with these viruses. When fluorescein-isothiocyanateconjugated convalescent sera obtained from mice initially infected with S-63 leukemia virus were used, the reactive site was within the cytoplasm of the infected cell. By electron microscopic examination, virus particles were demonstrated in the same areas within the cells that exhibited specific fluorescence with the conjugates. Spleen and mesenteric nodes contained the most virus, whereas kidney tissues contained the least amount. Fluorescence was not observed within the nuclei of infected cells.