Fragmented sarcoplasmic reticulum and transverse tubular system, as isolated in the microsomal fraction from rabbit skeletal muscle, bind H+, Na+, K+, Ca++, Mg++, and Zn++. The binding depends on a cation exchange type of interaction between these cations and the chemical components of the membranous systems of the muscle cell. The monovalent and divalent cations exchange quantitatively for each other at the binding sites on an equivalent basis. Scatchard plots of the H+ binding data indicate that the binding groups can be resolved into two major components in terms of their pK values. Component 1 has a pK value of 6.6 and a capacity for H+ binding of 2.2 meq/g N. The second component has a much higher H+ binding capacity (7–8 meq/g N), but its pK value, 3.4, is non-physiological. The binding of cations other than H+ at a neutral pH occurs at the binding sites making up component 1. The order of affinity of the cations for the microsome binding sites is H+ » Zn++ > Ca++ > Mg++ » Na+ = K+ as reflected by the apparent respective pKM values: 6.6, 5.2, 4.7, 4.2, 1.3, 1.3. Caffeine, which causes contracture and potentiates the twitch of skeletal muscle, does not interfere with the binding of Ca++ by the microsomes at neutral pH.