Studies were conducted to determine the feasibility of producing feline ovarian antiserum (FOA) in the rabbit and to characterize the antigenic composition of the feline ovary. Adult male rabbits were immunized by injecting an ovarian homogenate prepared from feline ovaries that had corpora lutea (CL) macroscopically removed. Using the agar-gel double diffusion (AGDD) and the immunoelectrophoretic (IE) techniques, 5 feline ovarian antigens were detected when feline ovarian extract was reacted with purified FOA (gamma-globulin fraction solution; protein content 24 mg/ml). The FOA reacted with 4 other feline reproductive organs tested, including the follicle, CL, oviduct, and uterus, to form 5 precipitin bands. The purified FOA also cross-reacted with feline serum and plasma and 10 other feline nonreproductive organ extracts tested to give 2 to 5 precipitin bands. Species cross-reactivity was also observed with FOA reacting with the rat ovary and the canine ovary, CL, and uterus, to form 1 to 2 precipitin bands. Immunoabsorption of the purified FOA by feline fever, kidney, or small intestine extract removed antibodies reacting with feline ovarian antigens. Precipitin bands failed to develop when such absorbed FOA were tested against the feline ovarian extract. Within the sensitivity limits of the AGDD and IE tests, organ-specific antigens of the feline ovary could not be detected. Feline ovarian antigens were located in isolated feline ovarian cells by the immunofluorescence technique. Fertility-regulating effects of the developed FOA were studied. One female cat given FOA during late gestation [gestation day (GD) 49 to 51] aborted 1 dead premature fetus on day 52 and delivered 3 live offspring on GD 64. Another queen was given FOA during midgestation (GD 36 to 40) and apparently resorbed 1 of 2 conceptuses. The FOA produced in the rabbit was not organ and species specific and may cause abortion or influence fetal development in the pregnant cats.