Previous research has demonstrated that dogs that received chemically abbreviated Dirofilaria immitis larval infections were significantly immune to challenge infections. Sera from those immune animals have been effective in passively transferring larval killing and stunting. In the present study, sera from immune and control animals were used to screen various Ag subsets for unique Ag. Through Western blot analysis of larval extracts and excretory-secretory products, and immunoprecipitation of metabolically labeled proteins and larval surface Ag, it was determined that as many as 12 molecules were uniquely recognized by protective immune sera. A 39-kDa molecule was present in both soluble lysates of third- and fourth-stage larvae and larval excretory-secretory products; it was recognized by each of the immune dogs and by none of the infected or uninfected control animals. The 39-kDa molecule appeared to be absent from adults and microfilariae of the parasite. In addition to the unique recognition by immune dog sera, larval stage specificity of this molecule suggests that it may be useful as a vaccine candidate.