Growth hormone-binding protein (GH-BP) levels were studied in cord serum of 69 human infants born after 24 to 41 wk of gestation and in serum of 14 infants aged 1 to 3 mo. GH-BP levels were measured by HPLC-gel filtration of serum incubated overnight with I-125-hGH. The radioactive elution profile revealed two small I-125-hGH peaks of high molecular weight and a large peak, corresponding to monomeric I-125-hGH. The first peak of high molecular weight was variable, showed some of the characteristics (high molecular weight, displaceability by a large excess of unlabeled hGH) of the described low affinity, high capacity GH-BP, and did not correlate with gestational age or birth weight (peak I-BP). The second peak was identified as I-125-hGH bound to the high affinity, low capacity GH-BP (peak II-BP). Mean +/- SD specific binding of I-125-hGH to this peak was significantly (p < 0.0001) different between preterm infants (3.1 +/- 1.1%; n = 51), term infants at birth (4.2 +/- 1.1%; n = 18), and 1- to 3-mo-old infants (8.5 +/- 1.6%; n = 14). To evaluate the effect of intrauterine nutritional state, the ponderal index (weight/length 3) was calculated. Peak II-BP levels were lower (p < 0.05) in infants with the ponderal index < 2.35 (2.8 +/- 1.0%; n = 20) than in those with the ponderal index between 2.35 and 2.65 (3.4 +/- 1.2%; n = 29) or > 2.65 (3.8 +/- 1.2%; n = 20). We conclude that GH-BP are present in human cord serum throughout the 3rd trimester of gestation. The levels of peak II-BP are influenced by fetal age and intrauterine growth and increase quickly after birth.