An assay is described for the detection and isolation of murine leukemia virus (MuLV)-infected cells in viable monolayers. The procedure utilizes antisera or monoclonal antibodies which specifically bind cell surface viral antigens of infected cells. Bound antibodies are subsequently detected by binding with 125I-labeled Staphylococcus aureus protein A followed by autoradiography of the tissue culture vessel. Focal areas of infection can be identified from the autoradiograph and infected cells can subsequently be isolated and subcultured as MuLV-producing cell lines. With appropriate antibodies the procedure should be useful for the direct isolation of minor components, including mutants, in complex virus mixtures. © 1983.