The modified Sherwood number relationship including the effects of property variations and suction by the membrane for laminar flow in a rectangular channel is presented in this work. The modified relationship is used to predict the permeate flux of ultrafiltration data of bovine serum albumin and dextran, taken from the literature. The exponent n of the correction factor (Sc/Sc-m)(n) was found to be 0.01 for bovine serum albumin for all feed concentrations; for dextran, n was found to vary with feed concentration (0.013 for 5% and 0.026 for 10% solutions). The analysis is valid for the imperfect membranes as well. Results are also presented for the simultaneous prediction of permeate flux and permeate quality (in terms of the observed retention). Finally, a parametric study of flux and observed retention is carried out to infer the influences of various operating and membrane performance parameters.