The present paper describes the use of [125I]Protein A, isolated from Staphylococcus aureus, in detecting antigen-antibody complexes by autoradiography on single cells. The method is relatively quick, reproducible, potentially more sensitive than immunofluorescence, and should be useful in combination with conventional radioimmuno-assays. We have used it to detect the cellular expression of IgM, kappa, lambda, and β2-microglobulin, as well as the expression of Epstein-Barr Virus (EBV)-associated antigens expressed in human lymphoblastoid cell lines. © 1979.