An improved method for the determination of cow's milk in non-bovine cheese is reported: electrophoresis of whey proteins in acidic, isoelectric buffers. Two background electrolytes (BGEs) have been tested: (i) 50 mM iminodiacetic acid (pH=isoelectric point=2.30 at 25 degrees C), 0.5% hydroxyethylcellulose, 0.1% Tween 20 and 6 M urea (apparent pH 3.1), E=300 V/cm, for the separation of alpha-lactalbumins (alpha-LAs); (ii) a BGE with the same composition, but supplemented with 10% Tween 20, E=450 V/cm, for the fractionation of beta-lactoglobulins (beta-LGs). Surfactants have a discriminating effect on the retention behaviour of the bovine alpha-LA and beta-LG proteins, owing to the different strength of the protein-surfactant association complexes, and are needed for separating these two proteins from small peaks in the electropherograms generated by degradation of casein during cheese ripening. Novel equations are given for deriving the ratio of the area (or height) of bovine alpha-LA, or beta-LG, to the area (or height) of ovine or caprine alpha-LA or beta-LG (such ratios being typically used to determine the percentage of cow's milk in dairy products), since previous equations had marked drawbacks, such as non-linearity of the plots with increasing slopes at high cow's milk percentages, and too broad confidence limits at high cow's milk contents, where the peak area (or height) ratio tends asymptotically to infinite. With the novel procedures reported, contents of cow's milk as low as 1% can be quantified in goat's and ewe's cheeses. The present protocols give lower detection limits, are cheaper and more rapid than any other methodology reported in the literature, and can be easily applied to the routine quality control of binary and ternary cheeses.