The antigenicity and allergenicity of so-called hypo-allergenic infant formulae is mainly determined by the degree of hydrolysis and ultrafiltration. Five different formulae were investigated by means of immunoblotting and RAST in order to characterize the antigens and allergens regarding their molecular weights, molecular origin and their ability to bind human IgG and IgE antibodies: A non hydrolysed infant formula (I-F), a mixture of the major cow's milk proteins (PM), a whey-based infant formula (W-H), a whey-based and ultra-filtrated infant formula (U-H), a casein/whey-based infant formula (CW-H). By immunoblotting we demonstrated that all tested formulae still contain antigens with molecular weights from 3 to 67 kD. But when compared with I-F and PM the antigen content of the hydrolysed formulae was considerably lower. The lowest antigen content could be demonstrated in U-H, which contains casein fragments (3-6 kD) and beta-lactoglobulin and its fragments (6-18 kD). W-H and CW-H contain bovine serum albumin, beta-lactoglobulin, casein and their fragments (3-67 kD). All hydrolysed formulae tested showed a reduced IgE-binding capacity. Three out of 12 cow's milk allergic children possessed IgE binding to U-H or W-H, and 5 of them IgE against CW-H. Conclusion. The enzymatic hydrolysis plus ultra-filtration seems to be the most efficient method to reduce the antigen content of so-called hypo-allergenic infant formulae.