The antigenicity/allergenicity of protein components in hypoallergenic formulae is altered by hydrolysis. Two different hydrolysate formulae, hydrolysate 1 = cow milk based; hydrolysate 2 = soya/beef collagen based, were investigated with respect to their specific IgG/IgE binding capacities using the sera of 41 healthy controls, 40 atopic and 12 cow milk allergic subjects. Furthermore, histamine liberation from basophils on incubation with milk proteins and the hydrolysates was measured in 5 healthy and 3 cow milk allergic individuals. Nearly all probands demonstrated specific IgG binding with hypoallergenic formulae. Anti-hydrolysate 1 IgG titres were higher than titres against hydrolysate 2 in the cow milk allergic and healthy populations. Nonhydrolyzed cow milk elicited IgE binding in all cow milk allergic sera. IgE-antibody for hydrolysate 2 could only be demonstrated in one atopic subject. Hydrolysate 1 showed binding capacity for the IgE-antibody of one atopic and 3/12 cow milk allergic patients. Histamine liberation followed in-vitro incubation with both hydrolysates in one out of 3 cow milk allergic subjects and two out of these cases following incubation to unprocessed cow milk protein. A decreased antigenicity/allergenicity can be demonstrated for the two hydrolysates investigated, however antigenic/allergenic reactivity is still present to some degree. Therefore, the therapeutic application of hypoallergenic formulae in patients with specific sensitization to cow milk should be based on the results of the above-mentioned in-vitro parameters and if necessary skin tests and oral challenges.