The content of insulin-like growth factor I (IGF-I) in human milk, cow's milk and cow's-milk-based infant formulas was determined by radioimmunoassay. The mean levels of IGF-I in full-term milk and preterm milk 0-4 days post partum 2.2 +/- 0.3 and 2.4 +/- 0.5 ng/mg protein, respectively. The IGF-I content in human milk was not affected by gestational age or birth weight and was constantly excreted up to 10 days post partum. The IGF-I content in human milk was significantly higher than that in cow's milk (0.6 +/- 0.1 ng/mg protein, p less than 0.01). IGF-I was not detected in the milk formulas. The IGF-I in human milk might be absorbed or could act locally in the intestine and may be of importance in the nutrition of neonates.