1. The cell‐membrane ATP phosphohydrolase of vegetatively grown Clostridium pasteurianum was specifically Mg²⁺‐dependent, but demonstrated significant activity with GTP, CTP and UTP. It displayed approximate Michaelis‐Menten kinetics only in the presence of certain effectors (e.g. phosphoenolpyruvate, fructose 1,6‐bis‐phosphate) which decreased the K_m, for ATP (to below 2 mM) but also V, whilst extending to pH 5.8 the effective pH range of activity of the enzyme.2. ATP phosphohydrolase activity of the membrane ATPase (BF₀F_l) was inhibited by N,N′‐dicyclohexylcarbodiimide, butyricin 7423, Dio‐9, 4‐chloro‐7‐nitrobenzofurazan, efrapeptin, leucinostatin and quercetin, and to a lesser degree by aurovertin and citreoviridin. The enzyme was not inhibited by oligomycin, spegazzinine, tributyl tin, triethyl tin or venturicidin. The soluble ATPase (BF₁) component differed in not being inhibited by N,N′‐dicyclohexylcarbodiimide, butyricin 7423 or leucinostatin.3. The ATPase (BF_oF₁) complex and its soluble (BF₁) component were separately purified.4. Dodecylsulphate/polyacrylamide gel electrophoresis separated only four polypeptide components in the purified ATPase (BF₀F₁), with approximate molecular weights (± 10%) as follows: subunita, 65500; subunit c, 57500; subunit d,_a, 43000; subunit f_a, 15000. The soluble (BF₁, component contained only the three polypeptide subunits a, c and d₂. These were present in the BF₀F₁ preparation in the ratio 2:1:2; the contribution of subunit f_a could not satisfactorily be quantified.5. Subunit a was identified as the component binding 4‐chloro‐7‐nitrobenzofurazan and sub‐unit f_a as the component binding N,N′‐dicyclohexylcarbodiimide. The ATP phosphohydrolase activity of the membrane ATPase was not activated by trypsin treatment and the ATPase (BF_oF₁) contained no trypsin‐sensitive inhibitor protein subunit.6. Purified ATPase (BF₀F₁) was incorporated into artificial proteoliposomes which demonstrated ATP‐dependent enhancement of 8‐anilinonaphthalene‐1‐sulphonate fluorescence and ATP‐dependent proton influx. These reactions were abolished by proton conductors (e.g. carbonylcyanide m‐chlorophenylhydrazone), by valinomycin in the presence of a high external concentration of K⁺, or by N,N′‐dicyclohexylcarbodiimide, butyricin 7423, Dio‐9, 4‐chloro‐7‐nitrobenzofurazan or leucinostatin. Oligomycin, tributyl tin, triethyl tin and venturicidin were not inhibitory.7. When stripped of the soluble BF₁ component, such ATPase‐proteoliposomes demonstrated nil ATP phosphohydrolase activity and did not display ATP‐dependent enhancement of 8‐anilino‐naphthalene‐1‐sulphonate fluorescence or ATP‐dependent protein influx. All of these activities were restored by incubation of the BF₁‐depleted proteoliposomes with a purified preparation of the soluble BF₁ component.

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