Orthovanadate at concentrations between 20 and 50 $\mu M$ markedly inhibits the DCCD—sensitive, $K^{+}$ stimulated ATPase activity of microsomal preparations from germinating radish seeds. The oligomycin-sensitive ATPase activity present in mitochondrial preparations (and, as a contaminant, also in microsomal preparations) is not inhibited by orthovanadate. These results are in full agreement with the recent data on the effects of vanadate on the ATPase activities of membrane preparations from Neurospora and suggest that vanadate is a useful, possibly specific inhibitor of the $K^{+}$, $Mg^{++}$ dependent plasmalemma ATPase.