The binding of insulin-like growth factor I (IGF I) on red blood cells has been studied in 13 children aged 8 months to 11 years and in 10 adults. The Scatchard analysis showed a curvilinear regression. In adults, the specific binding was 4.1% of the tracer, the mean number of high affinity receptor sites per cell (Ro1) being 0.88 (K1 = 10.74 nM-1) and the mean number of low affinity receptors sites (Ro2) per cell being 7.14 (K2 = 0.37 nM-1). In children the specific binding ranged from 3 to 6.5%. Ro1 ranged from 0.40 to 3.13 (K1 from 3.48 to 13.61 nM-1). Ro2 ranged from 2.88 to 17.25 (K2 from 0.03 to 0.65 nM-1). The most striking fact was the close positive correlation between the specific binding and the age of children (r = 0.914, P less than 0.001). These data suggest that the high growth velocity of young children, concomitant with the low plasma levels of IGF I which are physiological during infancy and early childhood, does not result from an increased binding of IGF I to cell receptors.