In 28 females, daily measurement of the HCG concentration in urine and in 15 of them daily measurement of the beta-HCG concentration in plasma was carried out during the first 2 weeks following first-trimester induced abortion by vacuum aspiration. Plasma beta-HCG concentration fell according to a multi-exponential curve with a half-life of 0.63 days in the first 2 days following induced abortion, and of 3.85 days in the subsequent 14 days. The disappearance of HCG from urine is exponential, with a half-life value of 1.3 days. A urine pregnancy test with a sensitivity of 1 IU/ml wil nearly always be negative in the course of 2 weeks after abortion. A positive test 4 weeks after abortion indicates an incomplete abortion or persistent trophoblast.