The monoclonal antibodies OC 125, OV-TL 3, MOv 18, and OV-TL 23, which are directed against distinct ovarian carcinoma-associated antigens, are assessed in the present study for their diagnostic value. The expression of the antigens defined by these was studied in serial sections from 80 ovarian carcinomas and 103 nonovarian carcinomas obtained from 183 patients, using the immunoperoxidase technique. Immunoelectron microscopy demonstrated that the antigens were all localized on the tumor cell membrane. OC 125, OV-TL 3, and MOv 18 reacted positively with approximately 90% of the ovarian carcinoma samples (n = 80), whereas OV-TL 23 stained 76% of these samples. OC 125, OV-TL 3, and MOv 18 were also reactive with a majority of carcinomas derived from the cervix, endometrium and fallopian tube, while OV-TL 23 reactivity was primarily restricted to ovarian carcinomas. OC 125 and OV-TL 3 reacted respectively with 11% and 14% of the nongynecologic carcinomas tested (n = 64). Positive cases included breast, colon, and lung carcinomas. The antibodies MOv 18 and OV-TL 23 were found less reactive with nongynecologic carcinomas.