The EPF activity of 67 sera, obtained during the 6th to 9th week of pregnancy, was determined using the rosette inhibition test. The sera of uncomplicated pregnancies (n = 9) and of women with habitual abortions who came to delivery during the most recent pregnancy (n = 10) contained the highest EPF activity during the 6th week of gestation. When pregnancies ended in another abortion (n = 10) the EPF was never detected, or disappeared at least 1-2 weeks before abortion. In the absence of EPF activity the relative risk of abortion was 7.6 (first determination) or 20.0 (second determination 1 week later). In comparison, the risk was lower when the more commonly used pregnancy parameters (HCG 4.4 (5.7), estradiol 2.6 (4.9), or progesterone (2.7 (2.7)) were depressed. Our results suggest that the EPF determination has high prognostic value in patients with high risk pregnancies and in pregnancies which follow treatment for sterility.