In this study, a detailed analysis of protein fractionation from casein whey using two-stage ultrafiltration (UF) with 30 kD and 10 kD flat-disk membrane in stirred rotating disk module has been made with an objective to understand the effect of membrane rotation and other important parameters on permeate flux and rejection. The membrane rotation was found to enhance the flux and was highly efficient in reducing the concentration polarization. The other independent variables being studied were solution pH, trans-membrane pressure (TMP) and stirrer speed. In the first stage UF with 30 kD membrane, most of the bovine serum albumin (BSA), lactoferrin (Lf) and immunoglobulin (Ig) were found to be rejected, and in the other with 10 kD membrane, alpha-lactalbumin (alpha-La), beta-lactoglobulin (beta-Lg) were rejected to give clear permeate with low protein concentration. To understand the effect of pH on whey protein UF, all the UF runs were taken at two different pH levels, 2.8 and 5.7, with 4.6 being isoelectric point (pI) of the casein. The effect of monomer-dimer equilibrium at higher pH has been observed and investigated. At feed pH 2.8 and membrane rotation at 3 00 rpm or more, higher fluxes were observed within the TMP range of 4-5 kg/cm(2). A 75.1% beta-Lg purity (on total protein basis) was obtained in the final stage, i.e. in 10 kD retentate at a TMP of 4 kg/cm(2) in stirred rotating disc module with 600 rpm membrane rotation speed.