Using the avidin-biotin binding system, an enzyme immunoassay procedure was developed to measure the membrane-associated placental tissue protein 1 (MP1) in serum. The standard curve covered the range from 10 to 1000ng/ml of MP1. the intra- and inter-assay coefficient of varations (C.Vs) were less than 5 and 10%, respectively. Recoveries of MP1 added to serum ranged between about 96 and 101%. The MP1 serum level was over 10 and under 112ng/ml in non-pathological men, and under 240ng/ml in non-pathological women. The MP1 level in the ovulatory phase was higher than in other phases of the menstrual cycle. In pregnancies during 6-39 weeks, the MP1 level ranged from 10 to 540ng/ml, and it increased during the third trimester of gestational age. In benign gynecologic diseases, the MP1 concentration in serum ranged from 10 to 215ng/ml. The MP1 levels in benign diseases were compared with those in ovarian malignancies, in endometrial carcinoma, and in uterine cervical cancer. The immunohistochemical location of MP1 was detected in the cell membrane of ovarian cystadenocarcinoma.