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A heterologous radioimmunoassay for the measurement of somatomedin C in sheep serum has been developed using purified¹²⁵I-labelled human somatomedin C and an antiserum to human somatomedin C. It was observed that the GH dependence of somatomedin C could not be verified when unprocessed sheep sera were assayed, because of interference by somatomedin binding proteins. After incubation of samples at pH 3·8, however, concentrations of somatomedin C mirrored GH status. Assay results from sera after prolonged exposure to acid agreed with results from samples which had undergone acidification and gel chromatography. Using the methods developed in this study, 12 sera from normal sheep had a mean concentration of somatomedin C of 2·35 ± 0·27 (S.E.M.) units/ml while nine sera from hypophysectomized sheep contained 0·45± 0·04 units/ml (P < 0·001), and one serum from a ewe with a prolonged GH excess associated with a pituitary tumour had a value of 6·9 units/ml. The radioimmunoassay resulting from these studies should provide a tool for investigation of the physiological control of somatomedin C in sheep.