[en] PLAP was isolated and purified from human placenta and the antiserum was obtained by immunizing the rabbits. A radioimmunoassay of PLAP (PLAP RIA) was established by labelling the antigen using the chloramine-T method. Its sensitivity was 1.54 μg/L, the recovery rate was between 96.7% and 105.2%, the intra- and inter-assay CV were 8.94% and 9.43%, respectively, the antiserum provided a linear response from 2 to 1000 μg/L. The assay has no cross-reactivity with liver AP. Serum level of PLAP were measured by PLAP RIA in 649 cases of normal pregnancy and 164 cases of high-risk pregnancy. The results indicated that the PLAP level increased proportionally with the advance of gestational age (r = 0.9843). In 33 cases of pregnancy induced hypertension and 21 cases of intrauterine fetal growth retardation, the PLAP were at significantly low level. In 7 cases of neonatal asphyxia and 26 cases of fetal distress, the PLAP level in the mother's serum were also low. In 53 cases of intrahepatic cholestasis of pregnancy, the PLAP level were similar to those of normal pregnancy. This study illustrated that PLAP RIA can play an important role in evaluation of placental function and fetal prognosis for cases of high-risk pregnancy