A hemagglutination test using tanned sheep erythrocytes was used to test serum of persons suspected of having visceral larva migrans, negative controls, contacts of suspected cases, and persons infected with various intestinal parasites. Cases of visceral larva migrans selected by clinical criteria usually (79.3%) showed titers for either Toxocara and/or Ascaris antibody above 1:80. Sera from over 90% of negative controls and contacts without signs of visceral larva migrans had low titers (1:80 or lower) with both antigens. There was little cross-reaction between sera from patients with hookworm, whipworm and visceral larva migrans, but sera from patients with strongyloidiasis frequently gave high titers with Toxocara antigen (20%) and with Ascaris antigen (51.4%). Patients having intestinal ascariasis but no signs of visceral larva migrans had low titers with Ascaris and Toxocara antigens.