Fluorescent precipitates were demonstrated on the cuticle and at the anal pore of Dirofilaria immitis microfilariae by using fluorescein isothiocyanate-labelled canine and human anti-D. immitis sera. Using fluorescein conjugated rabbit anti-human globulin, these reactions were shown to be antibody mediated. The precipitating antibodies were heat and 2-mercaptoethanol stable and induced specific passive Arthus reactions in guinea pigs when D. immitis antigens were injected intravenously. The titer of canine anti-D. immitis antibodies correlated with the microfilaremia of the serum donor, and the prevalence of human anti-D. immitis reactors in a community related to the numbers of infected dogs in the locality. Sera from people with pulmonary dirofilariasis gave high titers of anti-D. immitis antibody; rabbits injected with D. immitis antigens gave similar reactions, but both groups of sera cross-reacted with Toxocara canis larvae. Sera from individuals with T. canis visceral larva migrans, diagnosed from biopsied worms, did not react with D. immitis larvae. Concurrent gastrointestinal nematode infections in Aborigines did not affect the anti-D. immitis fluorescent antibody test.