The effect of bovine carbonic anhydrase (CA) on calcium carbonate (CaCO3) crystallization was investigated. A new method was developed to estimate the biocatalytic activity of CA in alkaline solution. The CA was immobilized within polyurethane (PU) foam, and compared its biocatalytic activity with the free-CA and bare-PU foam. A minireactor was created in a calcium hydroxide (Ca(OH)(2)) solution in order to control the CO, transfer rate, and reproducible results were obtained. It was found that the free-CA lost its activity in less than 6 min at pH 12.5 in alkaline Ca(OH)(2) solution. The CaCO3 crystallization rates for the immobilized-CA were found to be a U-shape, relatively lower at lower immobilized-CA amounts compared to those for the free-CA and bare-PU foam and higher when the amount of immobilized-CA increased. It was concluded that a higher immobilized-CA amount is required to accelerate the CaCO3 crystallization rates in Ca(OH)(2) solution.