SummaryThe indirect fluorescent antibody technique was used to detect antibody against Toxacara canis in 4 groups of sera. In a group of animals infected with T. canis or A. suum, T. canis antibodies appeared within 2 weeks of infection and a weak reaction with anti-Ascaris sera appeared after repeated infection. Sera from a group of rabbits and monkeys inoculated or infected with various bacterial, viral, and parasitic antigens gave no significant reaction. Five of 25 children, age 19 to 22 months, and 17 of 49 normal adult blood donors gave a significant reaction.