1. Delipidation of the Ca2+-ATPase of sarcoplasmic reticulum membranes by gel chromatography employing ionic detergents (cholate, deoxycholate and mixtures of both) in the presence of glycerol has been studied with respect to residual phospholipids and ATPase activities. 2. The extent of delipidation depends on the detergent chosen and on the ionic strength of the elution buffer. Increasing ionic strength favours a more effective removal of phospholipids, down to about 1 phospholipid molecule per ATPase molecule. 3. The residual ATPase activities of the delipidated preparations are negligibly low. Extensive restoration of the Ca2+-dependent ATPase activity has been achieved by oleic acid, a lysolecithin (myristoylglycerophosphocholine) and a lecithin (dimyristoylglycerophosphocholine). The percentage of reactivation by oleate depends linearly on the amount of residual phospholipids and on the detergent employed. 4. After gel filtration through an Ultrogel or Sepharose column containing 1% cholate in the elution buffer the delipidated ATPase is eluted as a reactivatable high molecular aggregate, whereas 1% deoxycholate favours the formation of completely lipid-free monomeric units which cannot be reactivated, however. A high molecular aggregate is also formed in deoxycholate, the ratio of monomer to polymer depending on the solubilizing and elution conditions. 5. The residual lipids are always composed of a mixture of all different lipid classes present in the native sarcoplasmic vesicles, even at high degrees of delipidation. Specific changes with varying extent of delipidation were not detected.