About a year ago one of us1reported six cases of chronic urticaria in which definite improvement was attained by the administration of a low sodium, high potassium, acid ash diet with the addition of from 4 to 6 Gm. of potassium chloride daily. Recently Bloom2has reported similar results in a variant group of allergic persons. This clinical success in a small series of cases prompted a study of possible chemical changes involved. The first requisite in our work was information as to the normal potassium content of blood serum. The values reported in the literature, however, show too wide discrepancies and variations to be uncritically accepted, especially in view of the fact that most of them were obtained by the method of Kramer and Tisdall,3the unreliability of which (and of its modifications)has been clearly pointed out by Van Slyke and Peters.4By means of an improved micro method (which is based on the gasometric determinations of a well defined mixture of potassium-silver-cobaltinitrite compounds), we were able to obtain results that agreed ±1 per cent with control determinations done by the macro platinic chloride method. The details of the method are to be reported shortly.