Recent reports that GH has a direct effect on growing cartilage have raised questions as to the role of somatomedins (Sm) in cartilage growth. To test the hypothesis that GH directly stimulates cartilage growth, we added homologous and heterologous GHs to organ cultures of embryonic chick pelvic cartilage. Pelvic rudiments from 9-day-old chick embryos were incubated in serum-free medium for 3 days in medium alone or medium containing chicken GH, turkey GH, bovine GH, human GH, and bovine GH produced by recombinant DNA methodology. None of the GH preparations studied stimulated avian cartilage growth in vitro. However, cartilage wet weight increased in response to sera from normal and growth hormone-treated hypophysectomized rats. In addition, 20 ng/ml purified Sm-C caused a 78% increase in cartilage weight above that of cartilage incubated in medium alone. Insulin also caused an increase in cartilage weight, but in concentrations 50,000-100,000 times that of Sm-C. Our studies demonstrate that homologous and heterologous GH have no effect on growing avian cartilage and support the hypothesis that Sm directly mediate cartilage growth.