We investigated the volume of maternofetal transfusion by measuring placental alkaline phosphatase (FLAP) as an indicator of placental passage. FLAP levels were measured in 135 pairs of maternal-neonatal sera. Estimated volumes of maternofetal transfusion were calculated with the following formula: Volume of transfusion (in milliliters) = 85 ml/kg x BW (in kilograms) x (PLAP(AB) - PLAP(1MO))/ PLAP(MA), where BW is the birth weight, PLAP(AB) is the FLAP level in neonatal serum at birth, PLAP(1MO) is the PLAP level in neonatal serum at 1 month of age, and PLAP(MA) is the PLAP level in maternal serum at delivery. The mean PLAP(MA), PLAP(AB), and PLAP(1MO) levels were 276.95 +/- 159.74 IU/L, 3.83 +/- 2.17 IU/L, and 0.25 +/- 0.22 IU/L, respectively, The mean volume of maternofetal transfusion was estimated to be 3.33 +/- 1.68 ml. The mean estimated volume of maternofetal transfusion per kilogram of birth weight was significantly lower in cases of scheduled cesarean delivery (0.74 +/- 0.35 ml/kg) than in cases of vaginal delivery (1.18 +/- 0.54 ml/kg; p < 0.001) and emergency cesarean delivery (1.73 +/- 1.00 ml/kg; p < 0.01). In scheduled cesarean delivery a significant positive correlation between gestational age and the estimated volumes of transfusion per kilogram of birth weight was observed. in cases of vaginal delivery, the estimated volume of transfusion per kilogram of birth weight was significantly Bower in the group with short labor (< 5 hours: 0.92 +/- 0.32 ml/kg) than in the group with prolonged labor (greater than or equal to 5 hours: 1.29 +/- 0.61 ml/kg; p < 0.001). PLAP was considered to be useful for estimating the volume of maternofetal transfusion. The transfer volume appeared to relate to uterine contractions and to histologic changes in the placenta with aging.