The peptide compositions of rabbit skeletal- and canine cardiac-muscle sarcoplasmic reticulum preparations have been compared by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate. The cardiac preparations contain many proteins in addition to the 105 000 dalton peptide which has been previously identified as the Ca2+ stimulated ATPase. Four peptide components iodinated in the presence of either free or Sepharose 4B-bound lactoperoxidase have molecular weights of 130 000 (component I), 105 000 (component II), 52 000 (component III) and 47 000 (component IV). Comparison of the labelling patterns in the presence of the detergent Triton X-100 suggests that components I, III and IV have part of their peptide internally located. Although part of component II is externally accessible to free lactoperoxidase, its iodination is decreased by Triton X-100. Iodination of phospholamban, the 22 000 dalton substrate for cyclic AMP-dependent protein kinase, was not observed under the conditions investigated.