The predictive value of various biochemical methods for monitoring early risk pregnancies has been compared in 65 cases of threatened abortion. Estimation of human chorionic gonadotropin (HCG), human placental lactogen (HPL), progesterone, oestradiol and alpha-fetoprotein (AFP) in serum were made by radioimmunoassays. Values below the normal range predicted abortion in 79, 81, 89, 92 and 38 per cent of patients, while normal values confirmed continuation of pregnancy with an accuracy of 71, 61, 60, 68 and 30 per cent respectively. Thus predictions from oestradiol and progesterone were at least as reliable as those from the protein hormones, while AFP proved to be unsuitable for this purpose. Combination of two variables gave even more reliable results. Due to individual and diurnal variation, however, abortion in the third and fourth month could not be definitely assumed at values above 5 IU HCG/ml, 5 ng progesterone/ml or 200 pg oestradiol/ml.