Binding of insulin-like growth factor I (IGF-I) to cultured resting, proliferative and hypertrophic growth plate chondrocytes was investigated. The optimal binding conditions and the extent of degradation of the 125I-IGF-I at 20 degrees C were analyzed in a time-course study. The maximal binding without noticeable degradation was observed after 3 h. The binding of IGF-I to the proliferative cells was 2-fold higher than to the resting and the hypertrophic cells. On the proliferative chondrocytes two classes of receptors with different affinities were found. 125I-IGF-I could be displaced from the proliferative cells by unlabelled IGF-I, IGF-II and insulin, respectively. Half maximal binding was observed at 0.3 nmol/l (= 2.2 micrograms/l) of IGF-I, 4.3 nmol/l (= 32 micrograms/l) of IGF-II and 350 nmol/l (= 2000 micrograms/l) of insulin. No specific binding of human growth hormone (hGH) could be demonstrated. When binding of epidermal growth factor (EGF) to the proliferative cells was assessed, little, but specific binding was observed.