Excretory-secretory (ES) antigens of adult Dirofilaria immitis were detected by enzyme-linked immunosorbent assay (ELISA) in serum and urine from each of 12 experimentally infected dogs. Excretory-secretory antigens in serum were first detected 154 days postinfection. Serum antibodies directed against parasite ES antigens were detected by ELISA in all dogs. Kidney tissue elution studies were performed in 10 dogs, and antibody and parasite ES antigens were demonstrated in each case. Antibody or parasite antigen was not detected in serum, urine, or kidney eluates from uninfected dogs. At peak concentrations of the ES antigens in serum, there were correlations with the number of adult D. immitis present in the dogs (r2 = 23.8, P less than 0.05) and with the antigen concentration in kidney eluates (r2 = 73.5, P less than 0.001). Peak serum antibody concentrations were not correlated with either the number of adult worms or the antibody concentrations in kidney eluates. This study suggests that detection of parasite antigens in urine may be an important diagnostic aid. In addition, the correlation between the concentration of D. immitis ES antigens in kidney tissue and in serum without a similar correlation between serum and kidney antibody concentrations suggests that D. immitis ES antigens adhere to kidney tissue.