Circulating Dirofilaria immitis antigen was detected in sera from 24 of 24 infected dogs by counterimmunoelectrophoresis (CIE). Parasite antigen was not detected in sera from uninfected dogs or dogs with Dipetalonema reconditum infection. In experimentally infected dogs, the antigen was first detectable 6.5–8.5 months after infection. Preliminary evidence suggests that the antigen is present in male and female adult worms but not in microfilariae. Sera from dogs with microfilaremic and amicrofilaremic infections contained statistically equivalent amounts of D. immitis antigen. However, a significant correlation was observed between serum parasite antigen content and the number of adult worms present in individual dogs at necropsy. Previous studies from several laboratories have shown that microfilarial counts and serum antibody titers are not related to adult worm counts in canine dirofilariasis or other filarial infections. Thus, CIE detection of D. immitis antigenemia represents a significant improvement over previously available diagnostic techniques because it is more sensitive than microfilarial tests, more specific than antibody tests, and the only test that has been related to infection intensity.