The molecular weight of the calcium-binding protein of rabbit white skeletal muscle was estimated to be 18500 by sedimentation equilibrium and electrophoresis in sodium dodecyl sulphate. 2. Addition of 2 Ca(2+) ions per molecule produced reversible changes in the u.v.-absorption spectrum that are interpreted as arising from conformational changes in the structure of the protein. 3. Cd(2+) was almost as effective as Ca(2+) in producing the spectral changes. Other bivalent metal ions, particularly Mg(2+), were less effective. 4. Binding of Ca(2+) by the calcium-binding protein produced an increase in mobility to the anode on electrophoresis in 6m-urea at pH8.6. The Ca(2+)-saturated form of the protein was more retarded on gel filtration than the Ca(2+)-free form. 5. In the presence of Ca(2+) the calcium-binding protein formed an equimolar complex with the inhibitory protein. This complex was stable in 8m-urea and in the pH range 7.0-8.6. 6. An isotope-dilution method for the measurement of the content of calcium-binding protein in whole muscle is described. In rabbit psoas muscle the ratio of actin monomers to molecules of calcium-binding protein was approx. 7:1. Similar values were obtained for red skeletal and cardiac muscle. 7. Evidence is presented indicating that in the rabbit the inhibitory protein of the troponin complex of red skeletal and cardiac muscles is different from the inhibitory protein of white skeletal muscle.
