Circulating levels of pregnancy-specific β1-glycoprotein (SP1 or PSβG), luteinizing hormone (LH) and human chorionic gonadotrophin (HCG) were measured serially in 9 subjects immediately after conception. Ovulation occurred spontaneously in 3 subjects, or followed administration of clomiphene citrate (2 subjects) or bromocriptine (4 subjects). The timing of ovulation was determined by the appearance of the LH surge. Levels of HCG were detected 10 to 16 days, and SP1, 18 to 23 days after ovulation. These findings suggest that the measurement of plasma levels of SP1 may provide valuable additional biochemical evidence of pregnancy.