introduction It has been known for some time that pituitary growth extracts stimulate the bony epiphyses of rats and that the increments in the width of the epiphyseal cartilage of hypophysectomized rats as results of growth hormone injections is proportional to the dosage (Li and Evans, 1948). This led Evans et al. (1943) to propose a simple bioassay method for estimating growth hormone potency. It was found that the method is highly sensitive and simple as compared to other techniques. In this paper we attempt to standardize the procedure and apply it to the assay of pure and crystalline growth hormone. preliminary experiments Prior to the standardization of the test procedure, certain experiments were done varying the route of administration, the duration of the test period, and the frequency of injections of hormone. In these experiments, both tibiae were removed 24 hours after the last injection, fixed in acetone, and stained immediately with silver nitrate, as described below.