The amino acid supply in an infant's "rst month of life must be su$cient in quantity and quality to ful"l the needs of this period of life. Guidelines, recommendations and minimum and maximum values have been established regarding to amino acid composition of infant formulas. The aim of this study was to determine the amino acid pro"le of infant formulas of di!erent types marketed in Spain in order to assess the compliance with the recommendations (ESPGAN) and to detect possible di!erences in the amino acid pro"les depending on the protein source. Amino acids were determined by HPLC in ion-pair reversed phase after hydrolysis and derivatization with phenylisothiocyanate. Nearly all the milk-based formulas analysed have higher amino acid contents than the values established by law, except for tryptophan. The soy-based infant formulas analysed have higher methionine contents in all cases than the value corresponding to the standard protein. Comparison of amino acid pro"les of infant formulas classi"ed according to their main protein fraction show some statistically signi"cant di!erences in the essential and semi-essential amino acids.