Ultrafiltration membranes of different hydrophobicities and different pore sizes were compared in the enrichment of alpha-lactalbumin (aLa) to the permeate from diluted whey solutions. The experiments were performed at low transmembrane pressures (TMPs), 10-30 kPa, in order to avoid heavy fouling. The fractionation was based mainly on the sizes of the proteins and on different forms of P-lactoglobulin (bLg) as the membranes were nearly uncharged. The highest selectivity was obtained with the hydrophilic 30 kg mol(-1) regenerated cellulose membrane and it was also the most resistant to fouling. Its aLa transmission increased from 10 to 26% when the solution temperature was increased from 25 to 40 degrees C. A 13-15-fold aLa/bLg ratio was achieved in comparison with initial whey in a single step at pH 6.5. However, the final aLa purity from all protein based products was only ca. 21% due to the high amount of small protein fragments. The selectivity of the 100 kg mol(-1) cut-off membrane decreased with temperature probably due to dissociation of bLg. The aLa transmission of the most hydrophobic membrane decreased steadily with time and TMP, but simultaneously its selectivity increased due to pore narrowing and higher retention of bLg. (C) 2009 Elsevier B.V. All rights reserved.