1. An oligomycin -sensitive ATPase was isolated and partially purified from beef heart mitochondria. The specific activity of ATPase sensitive to oligomycin
of the fraction was five to eight times that of aged mitochondrial or of DNP-induced mitochondrial ATPase assayed under the same condition. 2. Electron micrographs of the partially purified oligomycin- sensitive ATPase
reveal a structure in which headpieces are regularly attached by way of stalks to a thread-like structure derived from a superficial portion of base pieces. 3. A high concentration of the structured material coincided with a high activity of oligomycin-sensitive ATPase. When the headpieces were detached from the structure, the ATPase became insensitive to oligomycin. 4. The fraction of oligomycin -sensitive ATPase was essentially free of membrane structure and was contaminated with a small amount of cytochromes b and Cl but no cyt. a. Cytochrome concentrations of the preparations were indifferent to the activity of oligomycin sensitive ATPase. It follows that ATPase does not require cytochromes or membrane structure for its oligomycin sensitivity. 5. From these results it seems that the factor rendering ATPase sensitive to oligomycin should be contained in the stalks and/or the thread-like portion of basepieces of the structure. The structure is the simplest unit of oligomycinsensitive
ATPase as yet obtained. 6. The structure was called "oligomycin-sensitive ATPase particles" (abbreviated
as OSA particles). A unit of OSA particles consists of a headpiece attached by a stalk to a portion of base piece.</p
