Serum concentrations of PP5 were measured by radioimmunoassay in 219 women with normal pregnancies and 163 women whose pregnancies were complicated. PP5 in serum disappeared rapidly after delivery, with a half-life of 5–10 min in the first 10min. Serum PP5 levels were higher in uterine than in antecubital venous blood. In normal pregnancies, PP5 was detectable at 7–8 weeks of gestation; its mean concentration rose gradually to a maximum of 17.8 ± 10.2 ng/ml at 34–35 weeks of gestation. Elevated serum PP5 concentrations were noted in patients whose pregnancies were complicated by toxemia of pregnancy with appropriate-for-date baby or by twin pregnancy. Low serum PP5 concentrations tended to be found in patients whose pregnancies were complicated by abortion, intrauterine fetal death, and hydatidiform mole. Marked abnormal PP5 levels were not found in patients with maternal diabetes and placenta previa. These findings suggest that the assay of serum PP5 concentrations can be a useful parameter in determining the prognosis of abnormal pregnancies.