The binding of radiolabeled antiviral antibody to the surface of virus-infected cells can be used to detect viral antigens and measure antiviral antibody (1, 2). The binding of radiolabeled anti-γ globulin to unlabeled antiviral immunoglobulins on the surface of infected cells also can be used to quantitate antiviral antibody (3). This indirect technique is highly sensitive and has the advantage of employing a single reagent (125I-labeled anti-γ globulin) to detect antibody against a variety of viruses. Both the direct and indirect techniques, however, require fairly large quantities of materials and a monolayer of cells. We now describe an indirect radioimmunoassay which requires only small amounts of material and employs antigens prepared from virus-infected cells. Materials and Methods. Viral antigens. Monolayers of primary rabbit kidney cells, grown in Eagle's minimal essential medium with 10% heat-inactivated calf serum, were infected with type 1 herpes simplex virus (HSV) or vaccinia virus (CVI-79) (4–6).