In order to estimate the human placental lactogen levels and its value as an indicator of placental function during the first half of normal pregnancy, we defermined HPL levels. in normal snbjects (No=40) from the 6th week to the 20th week of gestation For the determination of human placental lactogen in serum of normal pregnant women during the first half of prognancy, radioimmunoassay have been performed using Phadebas HPL Test kit. The range of 25ng to 800ng HPL/ml. were determined by the procedures using Phadbas HPL Test kit, which correspond to levels recorded throughout the first half of normal pregnancy. HPL was detected in the earliest pregnancy samples collected at 6 weeks but was generally present in minute amount during the first trimester of pregnancy, when the HPL level ranged from 25 ng/ml. to 0.86 ${mu}g$/ml. The mean concentration of HPL increased gradually from only 0.03 ${mu}g$/ml, at 6 weeks to 0.25 ${mu}g$/ml, at 12 weeks of gestation, Subsequently the mean HPL value rose steeply from 0.25 ${mu}g$/ml, at 12 weeks to 1.65 ${mu}g$/ml, at 20 weeks of gestation.