A K(+)-stimulated adenosine triphosphatase was partially characterized in plasma membrane from meristematic and mature soybean root tissue. The substrate concentrations required for maximum enzyme activity (3 millimolar Mg.ATP) and pH optimum (6.5) were similar for both systems. Enrichment studies, performed to ensure that the membrane vesicle preparations were comparable, indicated similar purity levels at selected steps during purification. Phospholipid and sterol analyses further substantiated their similarity.Enzyme studies revealed significantly greater ATPase activity, per unit membrane protein, in the meristematic region. Mixing experiments indicated that the lower level of activity associated with vesicles from mature tissue was not due to endogenous inhibitor(s).