Peribacteroid membrane Mg++-dependent ATPase had higher activity at pH 8 than at pH 6 without K+. K+ stimulated the activity at pH 6 to a greater extent than at pH 8. Neither VO3- nor NO3- alone could inhibit the ATPase activity more than 40 % at either pH, but together resulted in a 52-63 % inhibition at either pH. These results were similar to those obtained using a mixture of plasmamembrane and Golgi membranes as enzyme source.