Calcium binding to troponin C and troponin was examined by a metallochromic indicator method under various conditions to obtain a further understanding of the regulatory roles of these proteins in muscle contraction. Troponin C has four Ca binding sites, of which 2 sites have a high affinity of 4.5 X 10(6) M-1 for Ca2+ and the other 2 sites have a low affinity of 6.4 X 10(4) M-1 in a reaction medium consisting of 100 mM KCl, 20 mM MOPS-KOH pH 6.80 and 0.13 mM tetramethylmurexide at 20 degrees C. Magnesium also binds competitively to both the high and low affinity sites: the apparent binding constants are 1,000 M-1 and 520 M-1, respectively. Contrary to the claim by Potter and Gergely (J. Biol. Chem. 250, 4628-4633, 1975), the low affinity sites are not specific only for Ca2+. The high and low affinity sites of troponin C showed different dependence on the ionic strength: the high affinity sites were similar to GEDTA, while the low affinity sites were similar to calmodulin, which has a steeper ionic strength dependence than GEDTA. Ca binding to troponin C was not affected by change of pH between 6.5 and 7.2. Troponin I enhanced the apparent affinity of troponin C for Ca2+ to a value similar to that for troponin. Trifluoperazine also increased Ca binding to troponin C. Troponin has four Ca binding sites as does troponin C, but the affinities are so high that the precise analysis was difficult by this method. The apparent binding constants for Ca2+ and Mg2+ were determined to be 3.5 X 10(6) M-1 and 440 M-1, respectively, for low affinity sites under the same conditions as for troponin C, being independent of change in pH between 6.5 and 7.2. The competitive binding of Mg2+ to the low affinity sites of troponin is consistent with the results of Kohama (J. Biochem. 88, 591-599, 1980). The estimate for the high affinity sites is compatible with the reported results.