The regularities of ultrafiltration (UF) of flexible chain polymers (FCP) differ from that of globular proteins (GP) which is manifested, in particular, in opposite dependences of retention coefficient (phi) on the applied pressure. According to one of the existing points of view this difference is caused by different influence of concentration polarization on these polymers and according to another by the deformation of polymer coils of FCP in longitudinal velocity gradients at the pore entrance. To settle this question we examined the behavior of globular proteins and dextrans under absolutely identical conditions, which was reached by UF of the mixture of these polymers in one experiment with separate registration of FCP and GP concentrations in permeate and bulk. As it follows from the experiments, one can make a conclusion that, at low pressures and also at low molecular masses (M), FCP molecules keep themselves as globular proteins i.e., as rigid spherical particles. However, with the increase in pressure and M, the transport of flexible chain molecules through pores of UF membrane begins to be determined by the mechanism of polymer chain uncoiling in longitudinal gradients.