Several applications of microcapsules for the encapsulation of living cells or macromolecules require well defined pore sizes. The molecular weight cut-off of alginate-polylysine microcapsules has been determined using a range of fluorescent labelled dextran molecules. The diffusion of the fluorescein isothiocyanate labelled (FITC)-dextrans into the microcapsules was followed by fluorescence and confocal laser scanning microscopy. The permeability of microcapsules for FITC-dextrans with a molecular weight of 4,700 daltons and the impermeability for FITC-dextrans with a molecular weight of 40,500 daltons was confirmed with both techniques. Determination of the molecular weight cut-off, using confocal laser scanning microscopy was more reliable and required a smaller sample than fluorescence measurements.