Studies were conducted to determine the feasibility of producing canine ovary antiserum in the rabbit and to characterize the antigenic composition of the canine ovary. Ovaries from dogs were obtained and corpora lutea (CL) macroscopically removed. Following homogenization of ovaries, adult male rabbits were immunized by injecting the ovarian preparation. Unabsorbed antiserum cross-reacted with canine ovarian extract and with other reproductive and non-reproductive organ extracts to form precipitin bands. Species cross-reactivity was also observed by testing the unabsorbed antiserum with extracts from organs of the cat and rat. Absorption of antiserum with canine liver and lung extracts removed antibodies not specific to the canine ovary. One band was observed when such absorbed antiserum was allowed to react with the canine ovarian extract. The absorbed antiserum produced no bands with extracts from other canine reproductive and non-reproductive organs tested. These experiments suggested that the canine ovary contained at least 1 organ-specific antigen. This organ-specific antigen was located in the isolated canine ovarian cells by the immunofluorescence technique.