It was found that injection of an antiallotype serum into the skin of a rabbit of a homologous allotype produced a skin lesion at the injection site. The skin lesions generated by this procedure were shown to result from the formation of insoluble immunocomplexes of the antiallotype serum injected and circulating IgG immunoglobulins: neither homocytotropic immunoglobulins fixed to cells nor surface immunoglobulins of B lymphocytes were found to take part in this reaction. The allotype skin test (AST) was utilized for identification of rabbit serum allotypes Aa1, Aa2, Aa3 and Ab5. A high degree correlation was found between results of typing by this method and by the precipitin test.