We have investigated the spatial variation of local protein concentration and filtration flux by a scintigraphic technique in the ultrafiltration of bovine albumin solutions and blood. The feed was mixed with (99m)Tc albumin macroaggregates and circulated through a polysulfone 30,000 MWCO hollow fiber filter placed in the field of a gamma-camera. Concentration profiles c(b) (x) were reconstructed from scintigraphic images and the local ultrafiltration flux was calculated by differentiating c(b) (x) and using mass conservation. Tests were run at various inlet shear rates from 472 to 1415 s(-1) and under two different filtration regimes: no net filtration (permeate valve closed) and large filtration (below the pressure independent plateau). The data confirm the filtration decay from the filter inlet to outlet but an unexpected result is the presence of high retrofiltration in the downstream part of the filter length in the case of large filtration. This retrofiltration can be explained by a high osmotic pressure at the membrane created by the protein polarization concentration. Assuming a constant pressure gradient along the fibers, it is possible to estimate the local osmotic pressure at the onset of retrofiltration and to infer from it the protein concentration at the membrane, which is found to vary from 170 to 250 g/liter when gamma(w) increases. Similar experiments were run with blood and a microfiltration membrane (0.55-µm pores). In that case no retrofiltration was obtained, which confirms our explanation since in this case the polarization layer is composed of red cells which exert no osmotic pressure.