Intracellular calcium concentration in biological cells varies from 0.1 to 10 muM depending upon cell signaling and disease states. A direct estimate of calcium concentration in cell tissues within this range is possible with a novel calcium-selective reagent 15C5-774. The molecule of 15C5-774 consists of a near-infrared (NIR) chromophore (lambda(max)=774 nm) and a metal complexing moiety of benzo-15-crown-5. The reagent shows a strong calcium binding affinity in a 1:1 ratio and metal selectivity in the order Ca(2+)>Mg(2+)>Sr(2+) approximately K(+) approximately Na(+)>Zn(2+)>Li(+). The high sensitivity is achieved by conducting absorption measurements in the NIR region where background interference from the biological matrix is low.