Tryptophan (TRP) is the limiting amino acid in low-protein infant formulas. This is mainly due to lower alpha-lactalbumin (alpha LA) content in cow's milk whey as compared with human milk protein. To study the effect of alpha LA-enrichment on the TRP supply, cross-over studies were carried out in 20 healthy infants up to 3 months of age. In this study, two protein-reduced (1.3%) infant formulas (moderate TRP content of 1.88% and higher TRP content of 2.10%) were alternately fed over a 2 week period in two groups of infants. Serum TRP levels of the formula-fed infants with the higher TRP content did not differ significantly from an exclusively breastfed control group of 11 infants (10.5 +/- 4.8 versus 10.9 +/- 4.7 mg l-1, p = 0.841), whereas levels of the formula-fed infants with the moderate TRP content were significantly lower (7.4 +/- 3.9, p = 0.038). The supplementation of alpha LA resulting in a higher TRP supply to low-protein diets is a further step towards the production of infant formulas more closely adapted to human breast milk.