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Pregnancy-specific β1-glycoprotein (SP1) levels in uncomplicated single pregnancies were measured by radio-immunoassay from the time of ovulation until the 8th week of pregnancy in 129 blood samples from 78 women. SP1 was detectable in all samples examined from 34 days after the last menstrual period (LMP), and thereafter the SP1 level increased rapidly with time and 38 to 40 days after LMP the geometric mean SP1 concentration was 90 μg/l. From pregnancy weeks 8 to 41, SP1 levels were measured by nephelo-metry in 1 255 blood samples from 1 255 women. A 95% reference range was established using logarithmic transformation. There was a steady increase in the SP1 concentration until the last month of pregnancy, in which a tendency to level off was seen, the geometric mean levels ranging from 149 to 170 mg/1. The day-to-day variation was studied during 5 consecutive days in 10 women. No significant variation was found. The diurnal variation was studied in blood samples taken every 4th hour during a 24-hour period form 10 women. A significant decrease was found at midnight and at 4 a.m. The elimination rate of SP1 from serum was studied in 10 women following labor. Elimination was non-linear and the time taken for SP1 to decrease to 50% ranged from 24 to 50 hours.No correlation was found between the concentration of SP1 and parity, maternal age, or the sex of the infant. The SP1 concentration was significantly lower in heavier women (> 70 kg) than in lighter women (≤ 70 kg). In some weeks of late pregnancy the SP1 level showed a correlation to placen-tal weight and birth weight of the infant.