PURPOSE: To reduce the effect of a temperature on a coloring sensitivity and to determine Ca in body fluids and the like with high sensitivity and accuracy, by specifying the composition of a colorimetric reagent for Ca, which comprises ortho-cresol phthalein complexon as a color former to obtain a linear calibration curve.

CONSTITUTION: A reagent comprising 2.8W3.6mg/dl of ortho-cresol phthalein complexon (OCPC), 80W120mg/dl of 8-hydroxyquinoline, 0.05W0.1mg/dl of Ca2+, 0.5W1.2mg/dl of boric acid, and 25W120mg/dl of monoethanolamine and having a pH of 9.8W10.5 is used in the colorimetric determination of Ca in body fluids and the like. A boric acid-KOH system is used as a buffer and the amount of monoethanolamine is minimized to enhance the coloring sensitivity, thus obtaining a high-sensitivity reagent having a linear calibration curve. In this way, accurate analysis, especially an automatic colorimetric analysis can be performed.