Summary: The β-subunit of chorionic gonadotrophin (/JHCG) was measured by radioimmunoassay in the serum of 190 consecutive patients admitted to hospital with the suspicion of ectopic pregnancy. The detection limit was set at 1 ng/ml. A urine sample was also taken for pregnancy testing on admission. A positive serum βHCG result was obtained in 36 patients (19%); pregnancy was confirmed in 32 (ectopic pregnancy 14, abortion 8, continuing normal pregnancy 7, retained products 3), giving a predictive value of 89%. There was poor correlation between serum and urine results. The serum levels were low in patients who subsequently aborted, intermediate in those with ectopic pregnancy and within the normal range in patients in whom the pregnancy continued normally. The high predictive value of the test should identify the patients for further investigation, thus avoiding unnecessary operative procedures and prolonged hospital stay.