A bioassay method has been developed to test for potassium deficiency in forest trees; it is based on the metabolic uptake by excised roots of 86Rb from 5 × 10−6 M rubidium chloride solution during a period of 15 min. Betula pubescens Ehrh. seedlings were grown in sand culture supplied with Hewitt's nutrient solution containing K at concentrations between 1 and 200 mg l−1. A negative curvilinear relationship was demonstrated between uptake by the seedlings of 86Rb from the rubidium chloride solution and the concentration of K previously supplied in the growth medium. When roots of intact plants were immersed in 86Rb-labelled solution for 15 min, less than 1% of 86Rb taken up was translocated to the shoots. A similar response curve, relating 86Rb uptake to K supplied to growing plants, was demonstrated when Picea sitchensis (Bong.) Carr seedlings were grown in peat culture with K added at 0 to 400 mg l−1. In three field trials of the bioassay, roots were excised from P. sitchensis and Pinus contorta Douglas ex Loud, growing on peat or peaty gley plots where different amounts of fertilizer had been added to give a range of degrees of K deficiency or from unfertilized, control plots. The patterns of 86Rb uptake by excised roots indicated that the method is potentially useful for the rapid determination of K deficiency in forest trees.