Rabbits infected orally with 20, 5 and 1 third-stage larvae of Gnathostoma procyonis were bled weekly for a period of 11 months. The sera that were collected were tested by an indirect tannedcell hemagglutination test with 5 extracts of adult worms and 2 of larvae. The adult worm extracts were found to be either nonreactive or nonspecific in their reaction. An acid-insoluble extract of larvae was also found to be nonreactive, whereas the acid-soluble fraction of the same preparation was found to be a sensitive, as well as specific, substance. The time interval between infection and the first detection of antibodies, the rate of antibody production and the maximum antibody titer noted were found to depend on the size of the inoculum. In all of the infections high titers were found to persist throughout the observation period. In two rabbits infected by placing larvae subcutaneously, the antibody response and the titer maxima did not correspond to the infecting dose. Sera from humans infected with schistosomes, hookworms and Ascaris did not cross-react with the acidsoluble larval extract. Similarly, no cross-reaction was noted with sera from animals infected with filariae, Clonorchis or Paragonimus. However, cross-reactivity was noted with sera from rabbits infected with Ascaris or Toxocara as well as with sera from human cases of tropical eosinophilia and visceral larva migrans. Conversely, no reaction was noted when sera from hosts with any of the above heterologous infections were tested with gnathostome extracts. The results of the infections seem to confirm the similarity between Gnathostoma spinigerum and G. procyonis.