The effects of an auxin herbicide, 2,4-D, at a concentration of 0.01 mM, on the K+ uptake and efflux of excised roots of wheat (Triticum aestivum L. cv. Rannaya) were investigated at different pH values. The K+ movement was monitored with a K+ (86Rb) tracer. In parallel experiments the ATPase activities of microsomal fractions were determined by the inorganic phosphate liberation method. 2,4-D inhibited the K+ uptake especially at low pH, irrespective of whether Ca2+ was present or not. No marked changes were observed in the K+ efflux properties at pH values above 4. The inhibitory effect on K+ uptake exhibited a correlation with the hydrocarbon solubility of the herbicide, but not with the 2,4-D-induced decrease of the ATPase activity. It is suggested that 2,4-D exerts a non-specific effect on the lipid-protein interactions, giving rise to a generalized alteration of the transport barrier properties of the plasma membrane even at as low a concentration as 0.01 mM.