The Mg2+ -dependent ATPase (EC 3.6.I.3) of Proteus L-form membrane has been solubilized according to various procedures (Tris - HCL shock-wash with or without MG2+, EDTA, Triton X-100). The best results were obtained by the same 33mM Tris-HCL (pH 7.5) shock-wash without MG2+ used for ATPase of protoplasts from Streptococcus faecalis. The solubilized enzyme after 105 000 times g centrifugation was purified on acrylamide/agarose. The molecular weight was established to be 360 000 by gel filtration and by sedimentation coefficient (12.5 S). Polyacrylamide disc-gel electrophoresis in sodium dodecylsulphate revealed two classes or subunit of mol. wt. 64 000 (alpha) and 58 000 (beta), associated in ratio 1:1. We propose a formula alpha-3beta-3 for the native ATPase of Proteus L-forms. Structural similarities to ATPase of various origins are discussed.