Cheese whey was concentrated 20 times by batch ultrafiltration using alternatively hollow fiber and spiral membranes of similar material (cut-off. 10 kDa) and the suitable process conditions that maximize the filtrate flux were studied. The resistance to filtration at bench scale was examined and modelled. The results showed that in the case of hollow fiber membranes the principal resistance is due to the fouling inside the membrane's pores, while the major resistance when using the spiral cartridge was originated by the polarization of the concentration. These results allowed the optimization of the operation and a significant improvement in filtrate flux. The mathematical models and the suitable operation conditions predicted by these models are presented. No difference was evidenced in the content of the concentrates obtained with hollow fiber or spiral membranes, both concentrates contained 99%, of the proteins of the original whey and showed 20 times higher coagulating activity than the original whey. © 2004 Elsevier Ltd. All rights reserved.