A study of whey ultrafiltration in a tubular membrane with a centrally inserted rod is reported in this work. Effects of the main operating conditions such as pH, temperature, and whey concentration on the ultrafiltration process have been investigated in a lab-scale tubular membrane. In particular, the mass-transfer resistance through the membrane (due to fouling and polarization), the flux decline vs. time, and the membrane-retention properties have been considered as representatives of the ultrafiltration process' performances. The study has been performed by using full factorial experiments, and results have been elaborated by the analysis of the variance (ANOVA). From permeability tests, the lowest value for the mass-transfer resistance (corresponding to the highest permeate flux) has been estimated at 5.8+/-0.3 MPa hr m(-1), under the following operating conditions: pH 6, temperature 25degreesC, whey concentration factor (CF) 1. The ANOVA suggested that pH and whey concentration were significant factors, while temperature resulted not to influence the mass-transfer through the membrane in the investigated range (25-40degreesC). Flux declines have also been monitored during time and profiles have been fitted by an empirical model. In this case, the highest temperature (40degreesC) favored the decline kinetics and it determined the highest difference between initial and stationary fluxes. As concerns the membrane-retention properties, the highest proteins-rejection coefficient (which means the highest retention) was 0.90 (under pH 4, temperature 25degreesC, CF 5). Furthermore, pH was demonstrated to affect significantly the membrane-retention properties. In fact, the chromatogram of permeates showed that some native proteins pass through the membrane at pH 6, while just peptides pass at pH 4.