The binding of five ¹²⁵I-labelled growth peptides (insulin, multiplication-stimulating activity (MSA), growth hormone (GH), epidermal growth factor (EGF) and platelet-deriyed growth factor (PDGF)) to chondrocytes isolated from rabbit and human cartilage, was examined. Total binding of ¹²⁵I-MSA and specific binding of ¹²⁵I-insulin I-EGF to human fetal chondrocytes was shown between 13W and 22W gestational ages, but at 14W and 15W, ¹²⁵I-PDGF did not bind specifically. Specific binding of all five ¹²⁵I-labelled peptides to post-natal rabbit (310g - 350g) chondrocytes was shown. For ¹²⁵I-MSA, ¹²⁵I-EGF and ¹²⁵I-PDGF, specific binding increased with advancing age of the rabbit (from 233g - 397g) which was coincident with an increase in rabbit growth rate. For ¹²⁵I-PDGF there was an increase in the number of binding sites/cell. Specific binding of ¹²⁵I-insulin to chondrocytes from rabbits of 1095g was 37% less than from rabbits of 305g. The lower binding was accompanied by a decrease in the affinity constant and in the number of binding sites/cell and occurred at an age when rabbit growth rate begins to decrease. Insulin (10 - 50μg/ml)increased ¹²⁵I-MSA binding to human fetal and post-natal rabbit chondrocytes by 300% and 20% respectively and somatomedin C (1000ng/ml) increased ¹²⁵I-PDGF binding to post-natal rabbit chondrocytes by 25%. Also, insulin at physiological levels (5 - 500ng/ml), increased the effect of plasma to stimulate [ ³H]-thymidine incorporation into post-natal rabbit chondrocytes by 54%, while insulin (5 - 500ng/ml), GH (1 - 5000ng/ml) and PDGF (10 - lOOng/ml) alone did not stimulate [³H]-thymidine incorporation. Growth peptides may therefore exert some of their growth-promoting actions in cooperation with each other and this may involve an increase in binding of one peptide by simultaneous binding of another. Specific binding of ¹²⁵I-insulin to human fetal chondrocytes in monolayer culture was also demonstrated but cell morphology was unstable and therefore binding measured may not have been truly representative of chondrocytes. It was concluded that the use of freshly isolated chondrocytes to measure growth peptide binding had more advantages.