Serum calcium, phosphorus, albumin, total protein, 25-hydroxyvitamin D (25OHD) and the vitamin D-binding protein (DBP) were measured in 30 cord sera and in 30 sera obtained simultaneously from their respective mothers. The maternal serum concentration of 25OHD (14.0 +/- 6.9 microgram/l, mean +/- SD) and of DBP (574 +/- 72 mg/l) were significantly higher than the respective cord serum concentration (8.0 +/- 4.4 microgram/l and 268 +/- 39 mg/l). The calculated concentration of "free 25OHD," however, was slightly but significantly higher in cord serum (0.44 +/- 0.24 ng/l) than in maternal serum (0.34 +/- 0.18 ng/l). Serum calcium and phoshporus were lower in maternal than in cord serum. A highly significant positive correlation was found between maternal and cord serum concentration of DBP (r = 0.59), total 25OHD (r = 0.79), "free 25OHD" (r = 0.86) and phosphorus (r = 0.73). These data indicate that the concentration of DBP is important for the evaluation of the placental transfer of 25OHD. Indeed, the concentration of "free 25OHD" is slightly higher in cord serum than in maternal serum, despite the maternal-to-fetal gradient of total 25OHD. The low fetal concentration of DBP is also unfavorable for the fetal storage of 25OHD during intrauterine life.