In a study of the effects of hydrocortisone on the embryonic chicken cartilage somatomedin assay, in the absence and in the presence of normal human reference serum (NHRS), it was found that: (1) The basal uptake of 35S into chicken embryo pelvic cartilage was reduced when hydrocortisone hemisuccinate was added to the incubation medium in concentrations ranging from 1.5 to 1.5 X 105 ng/ml. There was a correlation between the inhibitory effect and the quantity of hydrocortisone added (r = -0.869; p < 0.01). (2) The 35S uptake stimulated by 1.25 and 5 % serum present in the incubation medium was reduced by hydrocortisone in a final concentration range of 150–1.5 X 105 ng/ml incubation medium. The minimal dose was 1,000 times that required to affect the basal 35S uptake. (3) When hydrocortisone was directly added to the NHRS, its interfering effect on the 35S uptake stimulated by 1.25, 5 and 20 % of serum in the incubation medium was demonstrable with 5 X 105 ng hydrocortisone/ml serum. This concentration exceeded the physiological level of hydrocortisone by a factor of 5,00