The purpose of this study was to investigate modifications of the hemagglutination test that might improve its ability to detect antibody against Toxocara canis. Red cells, sensitized with antigens of Toxocara larvae. or infective eggs proved more sensitive as detectors of antibody than red cells sensitized with extracts of Toxocara adults. Ascaris adults, or Ascaris larvae. This greater sensitivity occurred when sensitization of red cells was done at 37°C, but not when done at room temperature. The sensitivity of red cells sensitized with extracts of Toxocara adults or various Ascaris stages was relatively low regardless of the incubation temperature during sensitization. Although cross-reactions were evident, the greater sensitivity achieved in the homologous system with red cells sensitized with antigens of the larval stage allowed better differentiation of homologous and heterologous antigen-antibody reactions.