Sarcoplasmic reticulum fragments, prepared from skeletal muscle homogenates, were found to consist of two major types when examined after negative staining. One type possessed 90 Å subunits and was thought to be of mitochondrial origin. The other had 35 Å subunits and ws presumably derived from the sarcoplasmic reticulum. Only the latter type accumulated visible calcium oxalate deposits inside the vesicle when they were exposed to a medium containing ATP, MgCl2, K2C2O4 and CaCl2. The calcium oxalate loaded vesicles were strikingly angular in shape and did not have tails. The calcium oxalate loaded vesicles had identical membrane subunit arrangement to inactive companion membranes and nonincubated controls; this suggested that the membrane subunits were not the critical structural requirement for calcium transport. A method was described whereby the calcium accumulating membranes could be purified 3- to 4-fold on the basis of the best previously used preparation procedures.