This paper describes the purification of solvent released turnip (Brassica napus L.) F1-ATPase. The specific activity of the purified ATPase was 135–194 μmol/min per mg when assayed in the presence of the activating anion, sulphite, or 40–55 μmol/min per mg in the absence of sulphite, values similar to those of conventionally isolated mammalian F1-ATPase. The isolated turnip F1-ATPase is cold-labile but can be stored in 30% v/v glycerol+4 mM ATP. Turnip ATPase (Mr 420 000) has six subunits with Mr 55 000, 52 000, 30 500 24 500 19 900 and 9100, respectively. Turnip ATPase has optimal activity at pH 8.0 and is more active with Ca2+ than Mg2+ as dependent cation. The calcium-dependent activity increases relative to the magnesium-ATPase activity when the ATPase is released from the membranes. Excess free Ca2+ stimulates whereas free Mg2+ inhibits turnip FI activity. The activity of turnip F1 is stimulated by chloride and markedly stimulated by oxy-anions, particularly sulphite. The nucleotide specificity in the presence of magnesium and sulphite is GTP>ATP>ITP>CTP>UTP; without sulphite the relative activities towards ATP and ITP are reversed. Turnip ATPase requires higher concentrations of classical ATPase inhibitors for inhibition than the bovine heart-muscle ATPase and is particularly susceptible to inhibition by the calmodulin antagonist Calmidazolium (R24571) in the presence of calcium.