In 15 women with suspected ectopic pregnancy considerable amounts of beta-subunit of human chorionic gonadotropin (hCG) were detected in the serum by the radioimmunoassay technique. The diagnosis was proved by laparotomy. In another 25 suspected cases beta-hCG values were less than 1 mU/ml, and in those cases the diagnosis was excluded. The detection of the beta-subunit of hCG in the serum was found to be an excellent tool for the early diagnosis of ectopic pregnancy, thus helping to prevent the dangerous sequelae which follow in the late diagnosis of this condition. The advantage of detecting the beta-subunit of hCG rather than hCG is discussed.