Antimicrofilarial immunity was studied in Dirofilaria immitis-infected dogs in order to better understand amicrofilaremic filariasis in man. Sera from dogs with amicrofilaremic infections contained IgG antibodies specific for microfilarial surface antigens detectable by immunofluorescence and in vitro leukocyte adherence. In vivo immune mechanisms were studied by injecting 51Cr-labeled microfilariae (MF) into infected and uninfected dogs. Injected MF were concentrated in lung, liver, spleen, and kidneys of normal and microfilaremic dogs, but circulated throughout the 5-hour study period. In contrast, injected MF were rapidly cleared (15-30 min) from the blood of amicrofilaremic-infected dogs. Tissue radioactivity and histopathology indicated that injected MF were trapped and destroyed in the lungs of these dogs. Antibody-dependent clearance and destruction of MF is a potent antihelminth effector mechanism in canine dirofilariasis. Similar events are likely to occur in amicrofilaremic filariasis in humans.