The purpose of the study was 1) to determine whether the primate Papio anubis responds to infection with Toxocara canis in the same way as man, and 2) to evaluate further the sensitivity of a microhemagglutination test by red cells sensitized with antigens of various stages of Ascaris or Toxocara. The white cell, antibody, and serum-protein responses of baboons given graded doses of Toxocara per os were monitored. Total white-cell counts reached a peak by the 2nd or 3rd week and returned to normal levels within 5 to 6 weeks after infection. The eosinophil counts rose and declined in similar fashion but remained above preinfection levels in most animals during the entire study period. Antibodies were detectable 1 week after infection by microhemagglutination tests with red cells sensitized with an extract of Toxocara larvae. Red cells sensitized with extracts of Toxocara adults. Ascaris suum adults, or larvae proved to be less sensitive detectors of antibody. Peak antibody titers occurred 2 to 3 weeks after infection, and antibodies were still detectable in high titers 6 to 7 months after infection. Generally, total serum proteins increased in all infected animals, reaching a maximum 2 to 5 weeks after infection. Most, if not all, of the serum protein increase was associated with the gamma-globulin fraction. It was concluded that the white-cell response of baboons to Toxocara either is inherently different from that in man, or else unknown factors associated with host sensitization are involved. The microhemagglutination test, with red cells sensitized with extracts of Toxocara larvae, appears to be useful in diagnosis.