Field-flow fractionation techniques including sedimentation field-flow fractionation (SdFFF) and flow field-flow fractionation (FlFFF) were applied to investigate hen egg white protein aggregation. The thermally induced aggregation of hen egg white protein was observed at temperatures of 60 degrees C and higher. Particle size and size distribution of hen egg white protein aggregates were characterized by SdFFF to investigate parameters affecting ZnCl 2-induced aggregation of hen egg white protein. At a fixed concentration of 1.0 M ZnCl 2 and an incubation time of 15 min, the mean particle diameters of the aggregates were determined to be 0.43, 0.67, and 0.80 mum for hen egg white protein contents of 5, 6.25, and 7.5% (w/v), respectively. With the incubation time of 15 min, increasing the concentration of ZnCl 2 from 0.5 to 1.0 and to 1.5 M caused the mean particle diameter of the aggregates to grow from 0.37 to 0.42 and to 0.68 mum, respectively at 5% (w/v) hen egg white protein. Upon prolonged contact time, larger aggregates were formed. Furthermore, FlFFF was employed as a novel approach to determine the efficiency of protein utilization for aggregation. The pH values as well as ZnCl 2 and protein concentrations influenced the efficiency of protein utilization for aggregation. With the optimum condition, that is, a protein concentration higher than 2% (w/v) and a pH greater than 5, the efficiency of protein utilization was approximately 65%.