The polarization sieving ultrafiltration (UF) model proposed earlier by one of the authors has been refined. According to this model, a gel that covers the membrane surface with a continuous layer during ultrafiltration reduces the radius of all membrane pores by a constant value Delta R, thus, in particular, explaining the preservation of the sieving mechanism of UF during gel polarization. To verify the validity of the assumption on invariability of Delta R, retention curves (RCs) have been calculated for membranes with different widths of pore size distribution at different levels of gel polarization. It has been found that the shift in the retention curves varies nonmonotonically and their width changes during the polarization, results that are inconsistent with the experimental data. It has been suggested that the pore size decrement due to gel polarization is proportional to the pore size (Delta R similar to R). The calculation of the retention curves based on this assumption has shown that the RCs undergo a parallel monotonic shift with increasing polarization, retaining their width, in agreement with the experimental results. These data suggest the mosaic character of gel polarization when the gel does not cover the membrane surface with a continuous uniform layer, unlike the case considered in the classical gel polarization theory, and is formed mainly at the pore entrance, and its local heterogeneous nature is determined by the dimensions of a particular pore size, rather than average membrane parameters.