A method for measuring somatomedin activity in vitro is described. Porcine costal cartilage fragments are prepared and incubated using a special dissection and incubation apparatus. The schedule consists of a 24 h pre-incubation, 46 h of incubation with the test substance and 3 h of post-incubation in a medium containing both 35S-Na2SO4 and 3H-thymidine. The average index of precision is 0.15 for 35S-uptake and 0.19 for 3H-thymidine. Although the assay is less sensitive than the hypophysectomized rat assay, it yields a considerable increase in precision, it is far less laborious to perform and is not so expensive.