The microfiltration (MF) fouling behaviour of beta-lactoglobulin solutions containing various levels of ionic calcium was investigated on a constant-flux, computer-controlled, cross-flow rig using zirconium oxide membranes. Fouling behaviour was highly dependent on both permeate flux and calcium concentration. At 2001/(m(2)h) on a 50 nm membrane the presence of 8.0 mmol/dm(3) (8.0 mM) ionic calcium in the feed increased fouling resistance dramatically. The presence of 8.0 mmol/dm(3) (8.0 mM) ionic calcium had little effect on protein transmission at 501/(m(2)h) but decreased protein transmission dramatically from about 90% to about 10% at 2001/(m(2) h). Most of the additional fouling observed in the presence of calcium was reversible, i.e. removed by water flushing. In runs commenced without ionic calcium, addition of 8.0 mmol/dm(3) (8.0 mM) ionic calcium after 1 h caused a rapid increase in fouling resistance and a rapid decrease in protein transmission. Similarly in runs commenced with 8.0 mmol/dm(3) (8.0 mM) ionic calcium, changing to a calcium-free feed after 1 h caused a decrease in fouling resistance and an increase in protein transmission. Presumably calcium was leached from the deposited fouling layer causing it to "dissolve". The two dominant reactions responsible for these effects of permeate flux and ionic calcium on fouling by beta-lactoglobulin solutions are probably molecular unfolding by shear and calcium crosslinking. (C) 2003 Elsevier Science B.V. All rights reserved.