A new serum assay for human chorionic gonadotropin, the radioreceptor assay (RRA), was compared with 3 commerical urinary agglutination inhibition pregnancy tests (AIT) in a group of women with suspected early pregnancies and ectopic gestations. The accepted definitions by which a laboratory test was characterized, including clinical sensitivity and specificity as well as predictive value of positive and negative tests and efficiency, were calculated for each test in 3 different time periods of gestation: < 7 days, between 7 and 14 days or > 14 days after the expected menses was missed. The RRA apparently had greater clinical sensitivity and efficiency than the AIT in cases of early gestation and ectopic pregnancy. The RRA had disadvantages of requiring the capability to handle radioactive materials and gamma counter. This test was chiefly useful for those patients desiring early termination of pregnancy and those infertility patients anxious for early confirmation of pregnancy.