In a comparative study six different protein hydrolysates, marketed as 'hypoallergenic' infant formulae were investigated by skin prick tests, RAST, RAST inhibition and titrated provocation tests. When hydrolysates containing a high percentage of larger peptides were found to have the highest capacity to induce positive skin tests, provocation tests and to bind to human serum IgE antibodies of cow's milk allergic children. Casein hydrolysates appeared to have the least residual allergenic activity. We recommend that 'hypoallergenic' formulae should be tested in each case, before being prescribed to cow's milk sensitive children.