Dextran sucrase has been produced by fermentation of Leuconostoc mesenteroides NRRL B-512, with and without continuous sucrose addition to improve enzyme production. The enzyme preparation has been concentrated from the fermentation broth by ultrafiltration and purified by gel permeation chromatography on Ultrogel. The specific activity of the dextran sucrase was greatly enhanced by calcium chloride addition to the purified enzyme. This enzyme preparation has been immobilized by covalent coupling onto an amino porous silica support (Spherosil) activated with glutaraldehyde. Immobilized dextran sucrase derivatives with an activity up to 830 dextran sucrase units per g. support could thus be obtained. The effect of the support specific area on coupling efficiency and reaction kinetics has been investigated, and the effect of intraparticular diffusion underlined. The molecular weight distribution of the dextran has been determined when varying several parameters.