Horseradish peroxidase-labeled antibody in combination with an in situ fixation and embedding procedure was used to locate viral antigens in infected tissue culture cells. The cytochemical reaction product mediated by the peroxidase-conjugated antibody allowed observation of antigen-antibody sites at both the light and electron microscope levels in the same preparation. Lymphocytic choriomeningitis (LCM) virus-infected 3T3 cells were studied. The electron microscope studies confirmed the morphology of the LCM virion as previously reported and showed that the extracellular virions were intensely stained by the enzyme-labeled guinea pig anti-LCM antibody conjugates. In addition, large cytoplasmic ribosomal aggregates were observed in the LCM-infected cells, and virus-specific antigens were demonstrated to be associated with these structures. Neither the cytoplasmic ribosomal aggregates nor the extracellular virions were stained with normal guinea-pig serum conjugated to horseradish peroxidase. The applicability of this technique for the study of in vitro virus-cell interactions is discussed.