Serum human placental lactogen (hPL) levels were measured in duplicate with a radioimmunoassay in 206 serum samples at 30 and 36 weeks'' gestation from women with normal singleton pregnancies (75) or pregnancies with twins (37). One triplet pregnancy was also studied. A significant elevation of hPL in the women with twin pregnancies at both the 30th (7.0 vs. 6.0 .mu.g/ml) and the 36th (9.2 vs. 7.4 .mu.g/ml) wk was found. Of the twin pregnancies 1/3 had values of hPL in excess of 8.0 .mu.g/ml at 30 wk and more than half had values in excess of 9.0 .mu.g/ml at 36 wk. The triplet pregnancy had an hPL value of 11.0 .mu.g/ml at 36 wk gestation. The potential usefulness of serum hPL measurements in the screening profile for the detection of high-risk pregnancies was discussed.