A bovine milk protein, beta-lactoglobulin (betaLG), was measured by an enzyme-linked immunosorbent assay (ELISA) in seven different infant formulas based on protein hydrolysates from cows' milk whey or casein, and from bovine collagen and soy. BetaLG levels in the formulas were 1/100 to 1/4 800 000 lower than in cows' milk (CM). There was a great difference in the betaLG level between the partly and the extensively hydrolysed formulas; the amount of betaLG was 40 000-fold higher in the partial hydrolysates vs the extensively hydrolysed formulas. Residual betaLG may have been responsible for the allergic reactions described in some children with cows' milk allergy (CMA) receiving these formulas.