Guinea-pigs have been given various preparations of cows' milk or infant formulae to drink in an investigation of the capacity of these milk preparations to stimulate per os anaphylactic sensitivity. The treatment, presumably heat, used to concentrate the 'Evaporated' whole cows' milks (three brands were tested) almost abolished their sensitizing capacity to beta lactoglobulin. The brand presumed to be most heated as judged by the degree of caramelization had also lost most of its sensitizing capacity to casein. Injected parenterally, the 'Evaporated' milk adequately sensitized to anaphylaxis. An infant formula, which in its spray dried form was only moderately sensitizing to both beta lactoglobulin and casein, lost most of this sensitizing capacity when processed to a liquid concentrate state. The manufacturing process for the liquid concentrate involved greater and more prolonged heating than required for the spray-dried form. The relevance of these findings in the guinea-pig to cows' milk protein intolerance in the infant and possibly also to cot death is discussed.