A bovine milk protein, beta-lactoglobulin (beta LG), 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. beta LG 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 beta LG level between the partly and the extensively hydrolysed formulas; the amount of beta LG was 40,000-fold higher in the partial hydrolysates vs the extensively hydrolysed formulas. Residual beta LG may have been responsible for the allergic reactions described in some children with cows' milk allergy (CMA) receiving these formulas.