The purpose of the study was to evaluate the compatibility of ozagrel sodium solution and calcium-containing transfusions using solubility product constants. We calculated the solubility product constant of mixtures of ozagrel sodium and calcium chloride and evaluated the compatibility of ozagrel sodium solution (both the original and generic products) with calcium chloride solution using a light obscuration particle counter. Various volumes of ozagrel solution were added to the calcium solutions to make final ozagrel concentrations of 0, 0.8, 1.6, 2.0, 2.4, 3.2 and 4.0 mmol/L. The solutions were gently agitated and stored at 25 and 40°C. The ozagrel concentration, calcium ion concentration and number of microparticles were measured. The solubility product constants obtained were 11.89×10(-9) mol(3)/L(3) (at 25°C) and 7.82×10(-9) mol(3)/L(3) (40°C). The number of insoluble microparticles was significantly increased when the ionic product was larger than the solubility product constant. In all ozagrel sodium products, the number of insoluble microparticles was within the allowable range according to the Japanese Pharmacopoeia. These results suggest that mixing ozagrel sodium with calcium-containing products is safe and without appreciable risk of incompatibility under clinical conditions.