An in vitro method for the assay of the biological activity of growth hormone is described. The method is based on the stimulation by the hormone of the incorporation of radioactive uridine into RNA in thymocytes from young rats. The response is dose-dependent and is linearly proportional to the concentration of growth hormone up to 16 μg/ml. Other pituitary hormones like FSH, LH, ACTH, TSH and oxytocin do not interfere with the assay system. Prolactin has some effect, which is, however, not additive to that of growth hormone. Various parameters of the procedure have been standardized. The index of precision of the method has been calculated to be about 0.05.