A serum assay was performed on 605 women for pregnancy-specific beta 1-glycoprotein (SP-1) between 31 and 34 weeks' gestation. SP-1 was also estimated for 255 of these women between 15 and 20 weeks' gestation, coincidental with maternal serum alpha-fetoprotein (AFP) screening. A clinical classification of pregnancy outcome was constructed for ech group. SP-1 values at 31 to 34 weeks' gestation in pregnancies that resulted in the birth of a small-for-dates (SFD) infant were statistically different from those in normal pregnancies (p less than 0.001). A value of 100 mg per liter was chosen as a suitable discriminatory level at 31 to 34 weeks, below which SP-1 had a sensitivity of 64%, a predictive value of 24%, and defined a relative risk of 5.3 for SFD infants. These parameters compare favorably with published data on the use of human placental lactogen as a screening test. No statistical relationship was found between SP-1 values at 31 to 34 weeks and birth weight, placental weight, parity, or other abnormal pregnancy outcomes. SP-1 screening at 15 to 20 weeks' gestation showed no correlation with serum AFP or with other parameters studied.