We have assessed the clinical utility of a rapid serum beta chorionic gonadotropin ( beta-hCG) assay. The assay employed a commercial kit that we have modified to assure its validity. We reach a sensitivity of 10 mIU/ml, and inter assay precision of 8,1% and an intra assay precision of 6.4 and 4.7% for concentrations of 7.6 +/- 0.5 mIU/ml and 1 825 +/- 86.3 mIU/ml respectively. This work was carried out on 106 radioimmune determinations of beta-hCG during the first trimester of 56 normal pregnancies and on 168 levels estimed in 116 suspected ectopic pregnancies. In normal pregnancies, a trophoblastic secretion of beta-hCG is detected on the 10th day following the thermal nadir; the levels quickly rise to reach a peak between the 42th and the 52th post-ovulation day, and then gradually decrease. In the 116 suspected pregnancies, 40 of them were surgically proven ectopic gestations: there were no false positive or false negative results. This rapid and accurate assay may contribute to the early diagnosis of an ectopic pregnancy.