Affinity electrophoresis in the presence of various triazine dyes of sheep milk proteins, including transgenically-introduced human alpha(1)-antitrypsin, has been evaluated as a predictive technique for possible large scale affinity-driven aqueous two-phase purifications. The success of the approach suggested it has potential as a general method for the rapid screening of ligands using only mu g amounts of sample, that could be applied to many complex mixtures before embarking on more costly and time-consuming two-phase partitioning experiments.