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 <sup>35</sup>S 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 10<sup>5</sup> ng/ml. There was a correlation between the inhibitory effect and the quantity of hydrocortisone added (r = -0.869; p < 0.01). (2) The <sup>35</sup>S 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 10<sup>5</sup> ng/ml incubation medium. The minimal dose was 1,000 times that required to affect the basal <sup>35</sup>S uptake. (3) When hydrocortisone was directly added to the NHRS, its interfering effect on the <sup>35</sup>S uptake stimulated by 1.25, 5 and 20 % of serum in the incubation medium was demonstrable with 5 X 10<sup>5</sup> ng hydrocortisone/ml serum. This concentration exceeded the physiological level of hydrocortisone by a factor of 5,000.