Because wet chemical methods generally are unsatisfactory, rubidium usually is determined hy spectrographic methods. Excitation of prepared samples of natural products by an electric arc or spark produce spectral lines of other elements that interfere with reading those of rubidium. This difficulty may be reduced by using flame excitation and a spectrograph of high dispersion. If the potassium concentration of the sample is high, as usually occurs in plant tissue, interference due to scattered radiation from intense potassium lines may be reduced by a mask, which prevents the energy from the potassum lines striking the spectrographic plate. Under proper conditions the method presented here is satisfactory for as low a concentration as 1 p.p.m. of rubidium in plant or animal tissue. The rubidium content of a number of human and animal foods was