Sera were collected from 237 patients from areas within the U.S. with suspected visceral larval migrans (VLM). Control sera were obtained from uninfected persons and from persons with Ascaris infection. The indirect hemagglutination test (IHA) was used to detect antibodies to Toxocara and Ascaris antigens. The antigens used were purified protein extracts obtained from adults and larvae of Toxocara canis and Ascaris serum. Nineteen of the 20 uninfected controls had negative IHA titers. Seven persons infected with Ascaris and without signs or symptoms of VLM were positive with both Ascaris antigens and negative with the Toxocara antigens. An adult dog infected with Toxocara canis was positive with Toxocara larval antigen only. Of the 237 persons with suspected VLM, 122 were negative with all four antigens and 115 were positive with one or more antigens. The specificity of the test was established by cross absorption with Ascaris and Toxocara antigens.