Chronic infection of L cells with Sindbis virus, induced in the absence of antiviral antibody, was studied by virological methods in combination with light and electron microscopy. Both Sindbis virus and endogenous oncovirus antigens were revealed in the cells by immunofluorescence. Sindbis virus and interferon were detected in the culture fluid. Immunoelectron microscopy revealed aggregation of Sindbis virus particles by immune serum.