The principle of integrated enzymatic synthesis and product separation is examined with the aid of an analytical ultracentrifuge. The biosynthesis of high molecular weight dextran polymer from sucrose using dextransucrase enzyme was chosen as the test reaction. Tests on pre-synthesised native dextran served to evaluate the process whilst allowing assessment of native dextran sedimentation properties. Native dextrans were found to have fully corrected sedimentation coefficients of 143-531 Svedbergs at infinite dilution. The biocatalytic formation of dextran was observed both visually and by measurement of the build-up of sedimented material in the ultracentrifuge cell. Sedimentation coefficients comparable to those observed for pre-synthesised dextran were recorded.