Insulin-like growth factor-I (IGF-I) plays a major role in cartilage homeostasis. Our objective was to study the penetration of IGF-I, both alone and bound to serum proteins, into the different zones of normal human cartilage using radioactively labeled IGF-I. The uptake of free IGF-I was higher than that predicted on the basis of excluded volume calculations and showed concentration dependence: we attributed this to reversible binding of the hormone to the tissue. Since the extent of binding was much higher than that calculated for binding to cell receptors, we concluded that IGF-I binds to matrix components. The kinetics of desorption of IGF-I from cartilage confirmed our conclusions regarding binding. The degree of uptake of IGF-I protein complexes prepared by labeling human serum with [125I]IGF-I showed that such complexes are largely excluded from normal cartilage and that the amounts present in the tissue are too low to affect proteoglycan metabolism.