Plasma membrane (PM) $H^{+}$-ATPase of sunflower (Helianthus annuus L.) hypocotyls with plasma membrane vesicles prepared by aqueous two-phase partitioning was characterized. Microsomal fraction proteins were partitioned into lower and upper phases using a polymer concentration of 6.4%. The ATPase activity was dependent on $Mg^{2+}$, but was slightly influenced by $K^+$. Molybdate, azide, and nitrate, which inhibit unspecific phosphatase, mitochondrial ATPase, and tonoplast ATPase, respectively, had little effect on the enzyme, reflecting the purity of the plasma membrane. However, $100{mu}M$ vanadate inhibited enzyme activity by more than 90%. $Ca^{2+}$ also increasingly inhibited enzyme activity with increasing concentration, and the inhibitory effect was considerable below pH 7. The ATPase had a $K_m$ value of 0.38 mM ATP, an optimum pH of 6.5, and an optimum temperature of $40^{circ}C$. Optimum removal of enzyme latency occurred with 0.01% Triton X-100 or 0.0025% lysolecithin, but Triton X-100 above 0.01% was inhibitory.