A stable and homogeneous adenosine-5ʹ-triphosphatase (ATPase, EC 18.104.22.168) has been solubilized from Rhodospirillum rubrum (R . rubrum) chromatophores by chloroform extraction. Purification of the Ca 2+ -dependent ATPase activity was 200-fold. Ca 2+ can be replaced by Mg 2+ , Cd 2+ , and Mn 2+ .The K m for Ca-ATP (0.17 mᴍ) is increased about 5-fold during solubilization of the enzyme, whereas the K m values for Mg-ATP (0.029 mᴍ) and Cd-ATP (0.014 mᴍ) are not affected. The chloroform-released ATPase has a molecular weight of 400,000 ± 30,000 and consists of the following subunits (molecular weights in parenthesis): α (58,000), β (53,500), γ (39,000), δ (18,500), and ε (14,000). The amino acid composition and the fluorescence spectra are presented. Besides the chloroform-released ATPase complex three other Ca 2+ -dependent ATPase forms have been isolated from R. rubrum chromatophores by other methods for comparison. Ultrasonication of the membranes leads to the release of an ATPase complex which is mainly composed of α, β, and γ-subunits. From an acetone powder extract an ATPase complex could be purified by affinity chromatography which is composed of four kinds of subunits (α, β, γ, δ). The same acetone powder yields an ATPase consisting of only three different types of subunits (α, β, γ) if the final purification step is preparative disc electrophoresis on 6% polyacrylamide gels instead of affinity chromatography.