In dogs experimentally infected with Dirofilaria immitis, antibodies were not detected by complement fixation (CF) before the 3rd month, whereas with indirect hemagglutination (IHA) antibodies were detected as early as 2 weeks after inoculation. The overall pattern of a progressive rise and fall in antibody level was basically the same with the CF and the IHA tests and after the 9th month anitbodies could not be detected with either test or with any of a number of different antigens. IHA with an acid-soluble protein fraction was the most sensitive test system used. Although this antigen was also the most specific, it reacted in low titers with sera from rabbits infected with Ascaris lumbricoides or Toxocara canis and from humans infected with Schistosoma japonicum or T. canis.