Raw water and aqueous solutions processed by membrane filtration usually contain metal ions or colloids to some extent. In this work, we studied the effects of the ionic environment and metal colloids on the protein fouling of hollow fiber ultrafiltration membranes. The addition of calcium ions to a feed protein solution caused severe casein fouling, but no change in BSA or lysozyme fouling. The addition of FeCl3 to a feed BSA solution caused severe fouling, even at 0.05 mM FeCl3, while lysozyme fouling was relatively unaffected by the addition of FeCl3. Field emission-scanning electron microscopy and energy dispersion spectroscopy revealed the deposition of BSA and iron on the fouled membranes. Dynamic light scattering indicated that casein aggregation was induced by calcium ions, and that a BSA/iron colloid complex formed in the presence of FeCl3; both of these factors caused increased fouling. The addition of dithiothreitol to a BSA/FeCl3 solution suppressed the fouling to some extent, probably because of the decomposition of the BSA/iron colloids complex. (C) 2013 Elsevier B.V. All rights reserved.