Cow's milk proteins are amongst the most common causes of food allergy in infants, and caseins are probably the main allergens. The existence of a high degree of cross-reactivity between milk caseins from different animals has been reported. We describe a 2-year-old boy who experienced allergic reactions after eating and touching sheep's cheese, but who tolerated cow's milk and cow's milk dairy products. He had never ingested milk or milk derivatives from sheep or goat.Skin prick tests were carried out using whey fractions of cow's milk, whole milk and casein from goat, sheep and cow. We also performed skin prick tests with enzymes used in cheese production. Prick-by-prick tests with cheese made from cow, sheep and goat and their corresponding whole milk were also performed. Total serum IgE and specific IgE to cow's milk proteins, whole cow's milk and sheep's milk were determined. Specific IgE against casein and whole milk from the three different species were determined by ELISA. Inhibition of IgE binding to bovine casein was tested for casein and whole milk from all three species. The proteins of three types of casein and whole milk from cow, sheep and goat were separated by SDS-PAGE and were incubated with the patient's serum.Skin tests were positive to sheep's milk and goat and sheep casein and were negative to all cow's milk proteins and whole cow's and goat's milk. Prick-by-prick tests were positive to goat's and sheep's cheese and were negative to cow's cheese. In ELISA-inhibition, sheep's milk and goat and sheep casein were able to inhibit > 50 % of specific IgE binding to sheep casein. The results of immunoblotting showed that the patient's circulating IgEs recognized only one band in the lanes corresponding to sheep and goat casein.We report a patient with allergy to sheep's and goat's milk proteins but not to cow's milk proteins. Sheep casein was probably the main allergen causing sensitization in this patient. The results suggest that sheep casein shows a high degree of cross-reactivity with goat casein but not with cow casein. Our patient presented allergic symptoms caused by sheep and goat milk and cheese proteins. However, he was able to tolerate cow's milk and cow's milk dairy products without any ill effects.