This paper presents experimental evidence supporting the theory of restricted diffusion and molecular sieving through porous cellulose membranes, developed by Pappenheimer and his coworkers. The study measured ultrafiltration rates, molecular sieving during ultrafiltration, and diffusion rates of various molecular species through inert porous membranes. The results were compared with predictions based on the theory. Estimates of membrane pore radii and diffusion areas per unit path length were found to be consistent with the theory and in contrast to estimates from the Elford and Ferry calibration method, which were found to be inaccurate. The experiments involved measuring the diffusion of tritium-labelled water, urea, glucose, antipyrine, sucrose, raffinose, and hemoglobin through three types of cellulose membranes. The diffusion rates were calculated using Fick's law. Ultrafiltration experiments were conducted using water and aqueous solutions of various solutes, and the sieving effect was quantified by the sieve coefficient (c₂/c₁). Additional physical measurements included membrane thickness and water content. The study also examined the relationship between molecular size and diffusion area, and found that the apparent diffusion area decreases with increasing molecular weight. The theory of Pappenheimer et al. was found to accurately predict these results, and the estimated pore radii and diffusion areas were consistent with the theory. The results also showed that the permeability of cellulose membranes differs from that of living capillary endothelium, with the latter providing additional diffusion pathways for lipid-soluble substances. The study also investigated the osmotic pressure exerted by solutions during molecular sieving and found that the measured osmotic pressures were close to the ideal values predicted by van't Hoff's law. The results support the methods and theory used by Pappenheimer et al. in their studies on living capillary walls. The paper concludes that the theory of restricted diffusion and molecular sieving through porous membranes is valid and that the new method of membrane calibration based on direct diffusion of isotope-labelled water is more accurate than the Elford and Ferry method.This paper presents experimental evidence supporting the theory of restricted diffusion and molecular sieving through porous cellulose membranes, developed by Pappenheimer and his coworkers. The study measured ultrafiltration rates, molecular sieving during ultrafiltration, and diffusion rates of various molecular species through inert porous membranes. The results were compared with predictions based on the theory. Estimates of membrane pore radii and diffusion areas per unit path length were found to be consistent with the theory and in contrast to estimates from the Elford and Ferry calibration method, which were found to be inaccurate. The experiments involved measuring the diffusion of tritium-labelled water, urea, glucose, antipyrine, sucrose, raffinose, and hemoglobin through three types of cellulose membranes. The diffusion rates were calculated using Fick's law. Ultrafiltration experiments were conducted using water and aqueous solutions of various solutes, and the sieving effect was quantified by the sieve coefficient (c₂/c₁). Additional physical measurements included membrane thickness and water content. The study also examined the relationship between molecular size and diffusion area, and found that the apparent diffusion area decreases with increasing molecular weight. The theory of Pappenheimer et al. was found to accurately predict these results, and the estimated pore radii and diffusion areas were consistent with the theory. The results also showed that the permeability of cellulose membranes differs from that of living capillary endothelium, with the latter providing additional diffusion pathways for lipid-soluble substances. The study also investigated the osmotic pressure exerted by solutions during molecular sieving and found that the measured osmotic pressures were close to the ideal values predicted by van't Hoff's law. The results support the methods and theory used by Pappenheimer et al. in their studies on living capillary walls. The paper concludes that the theory of restricted diffusion and molecular sieving through porous membranes is valid and that the new method of membrane calibration based on direct diffusion of isotope-labelled water is more accurate than the Elford and Ferry method.