A passive cutaneous anaphylaxis test (PCA) for determination of the allergenicity of bovine whey proteins and peptides was developed in mice. Antisera against whey proteins raised in rabbits and in mice, using a procedure for high IgE titre mixed with Freund's incomplete adjuvant, were applied intradermally, and various whey proteins and whey protein hydrolysates were tested for positive PCA reaction. Unhydrolysed whey and peptides larger than 6500 Da were found to react positively, peptides between 6500 Da and 3400 Da reacted weakly, whereas peptides smaller than 3400 Da were unable to initiate a reaction. The studies indicate that guinea-pigs, widely used by tradition, may be successfully replaced by mice in determination of the allergenicity of various compounds. The sensitivity of the two species is similar, but the husbandry and handling of mice is more convenient, and they are less expensive.