Purified rabies virus was labeled with 125I, and preparations with specific activities of 4.4 to 12.7 × 107 cpm/mg of viral protein were obtained. The labeled virus was reacted with dilutions of rabies antibodies and the resulting virus-antibody complexes were precipitated with anti-IgG serum. The specific binding activity for each serum dilution could be calculated after subtraction of the background activity determined from samples of normal serum treated in the same manner. When sera from immunized humans and rabbits and a serum sample from a patient with a clinical rabies infection were evaluated by this procedure, the virus-binding ability of anti-rabies serum was shown to be similar to or higher than the values obtained by the neutralization test based on plaque reduction method. The radioimmunoassay procedure was also used to determine the proportion of rabies-specific γ globulins in a mixture of 131I-labeled γ globulins from immunized rabbits and 125I-labeled γ globulins from normal rabbits. Ten to twelve per cent of all γ globulins represented rabies-specific antibody in this serum sample.