Abstract We raised an antiserum (Tr4) in rabbits against a basic somatomedin C-like peptide preparation. Using high-immunoreactivity somatomedin C tracer, we compared the performance of radioimmunoassays in which we used the Tr4 antiserum and a well-characterized somatomedin C antiserum distributed by the National Pituitary Agency (NPA) with that of the human placental-membrane somatomedin radioreceptor assay (RRA). In their cross reactivity toward various somatomedin-like and unrelated peptides, the two radioimmunoassay methods were almost identical, although NPA antiserum, with about fourfold higher titer than Tr4 antiserum, showed a slightly greater sensitivity for most peptides tested. Radioimmunoassay of acid-ethanol-extracted plasma samples from normal persons and acromegalic, hypopituitary, hypothyroid, and renal-failure patients revealed no analytical differences between the antisera (for 122 samples, r = 0.979 between methods). Somatomedin values for acromegalic and hypopituitary samples showed no overlap with normals. Values for hypothyroid and pre-dialysis renal-failure samples were significantly lower than normal. By comparison, the RRA showed greater cross reactivity toward some somatomedin-like peptides and gave significantly lower values than radioimmunoassay for acromegalic and hypothyroid plasma extracts, and significantly higher value for hypopituitary and renal-failure samples. We conclude that the radioimmunoassay methods clearly are of greater diagnostic value than RRA for clinical somatomedin measurement.