Analytical gel filtration was used for the study of molecular size distribution of clinical dextran in serum and urine for the purpose of evaluations of changes in the human glomerular barrier function. The column was calibrated in terms of solute size using a simple and accurate technique recently described. Only one sample of a dextran possessing a broad molecular mass distribution was necessary for the calibration procedure and the calculations were performed using an ordinary spreadsheet. The accuracy of the calibration, as evaluated by protein samples, is better than 95%. The simplicity makes the method suitable for use in laboratories not normally specializing in analytical gel filtration. Calibration in terms of size is preferably done with respect to viscosity radius to obtain relevant information about the permeability of dextran into porous membranes.