A purified preparation of ATPase (factor F1) from the Acholeplasma laidlawii was obtained. The purification procedure included extraction of the enzyme complex from the isolated membranes by ultrasonication, chromatography on DEAE-cellulose and gel filtration on Sepharose 6B. The specific activity of the ATPase was increased 30-fold as compared to the original activity. The Km value for ATP hydrolysis was 7,4 . 10(-4) M. ADP competitively inhibited the enzyme (Ki = 2,0 . 10(-4) M). Ouabain (2,5 . 10(-4) M) and dicyclohexylcarbodiimide (1,0 . 10(-4) M) did not inhibit the ATPase activity. The enzyme was activated by Mg2+, but was inhibited by a combination of Na+ and K+. The enzyme is cold-labile, but can be stabilized by storage in buffer solutions, containing methanol, glycerol or lecithin.