Four dogs were immunized against Dirofilaria immitis infection by a series of 3 larval infections which were each subsequently terminated by ivermectin treatment. Two control dogs received ivermectin treatment alone. Following the final ivermectin treatment, dogs were challenged with infective larvae by subcutaneous inoculation, both free and contained within diffusion chambers. Three weeks after larval challenge the chambers were removed and live larvae were enumerated. Seven months after challenge dogs were killed and necropsied to collect and count adult D. immitis. Chambers recovered from immunized dogs had 63% fewer larvae than chambers from control dogs. At necropsy, control dogs had a mean of 28.5 adult worms whereas the immunized animals had an average of 0.5 worms (range 0–2). Sera collected from immune dogs throughout the study had elevated antibody levels to third- and fourth-larval stage antigens. Significant levels of immune protection were achieved with this immunization regimen. The data suggest that a multiple-stage parasite killing occurs in immune animals. It was not possible to associate immune protection with any of the 5 antigen subsets.