Serum TeBG binding capacity was measured in 8 cord blood samples, and in peripheral venous blood from 39 male and 31 female infants aged from 1 day to 1 year. In cord blood, TeBG binding capacities were low (1.27 +/- 0.3 microgram/100 ml) with no sex difference. In male infants, TeBG binding capacities increased progressively from birth to 3 months, before decreasing to reach the normal prepubertal level at 6 months. Individual values ranged between 1.16 and 14.5 microgram/100 ml and were significantly correlated with plasma testosterone (r = 0.671, 95% confidence limits 0.440 to 0.818, p less than 0.005) and estradiol (r = 0.734, 95% confidence limits 0.121 to 0.942, p less than 0.01) levels. In female infants, individual values ranged between 1.17 and 14.5 microgram/100 ml, without correlation with age or plasma estradiol level. In male infants the data suggest a positive control ot TeGB binding capacity by estrogens, the negative effect of testosterone being delayed until after the 3rd month of age. In girls, the lack of correlation between TeBG and estradiol can probably be explained by rapid variations of plasma estradiol levels.