Heartworm disease caused by Dirofilaria immitis affects canine and feline hosts. Moreover, the parasite can infect humans, causing pulmonary dirofilariosis. Most affected dogs have patent infections with circulating microfilariae in peripheral blood, although infected dogs sometimes develop occult infections characterized by the absence of microfilariae. Microfilaremic infections (mf+) are associated with a predominant Th2-type immune response, whereas a Th1-type response predominates in amicrofilaremic infections (mf-), suggesting a role for this response in the suppression of circulating microfilariae. However, nothing is known about the molecules involved in the immune regulation of these infections. The objective of the present work was to identify the parasite proteins recognized differentially by the immune response of dogs with patent or occult infections, using two-dimensional electrophoresis and mass spectrometry. Nineteen proteins of D. immitis were identified, of which 6 were immunoreactive against serum samples from both mf+ and mf- dogs, while another two groups of 6 and 7 different proteins were differentially recognized by sera from mf+ or mf- dogs, respectively. The results point to the existence of differential antigen recognition in patent and occult infections due to D. immitis. Several proteins that could be involved in the immune regulation of these infections are identified. Additionally, the findings seem to suggest that some antigens of D. immitis, together with Wolbachia antigens, could contribute to the stimulation of the Th1-type response.