I. A method was devised which was accurate, simple and rapid. Using the hypophysectomized frog, Rana pipiens, as the test animal, it was found essential to control the pH and to compare the known and unknown at the same pH. An important finding was the fact that an accurate assay could not be made by comparing the vol. of soln. as a variable. It was essential to compare varying concs. used in the same vol. The assay as described eliminates the tedious and impractical procedure of attempting to dilute a known and unknown until they give similar responses.[long dash]II. The melano-phore hormone in well buffered solns., over a wide pH range, underwent potentiation to some degree below a pH of 8 but above this value there was a continuously increased potentiation as the pH rose. No break was found in the curve from the plotted data. No specific inhibitory sub-stance was present in the gland, the destruction of which might account for the increased activity. The evidence indicates that potentiation is an alteration in the nature of the hormone molecule.