This study was designed to evaluate free amino acid (FAA) composition and total protein in mature human milk from Turkish mothers. Free amino acid concentrations in mature human milk were determined in all subjects using a high-performance liquid chromatography (HPLC) with postcolumn derivatization system, with a fluorescence detector. Total protein content was determined by the classical biuret method. Total protein concentration was found to be 1.3 +/- 0.4mg/dl. Glutamic asid plus glutamine is the most abundant amino acid (1275 mu mol/L), followed by taurine (353 mu mol/L) and alanine (261 mu mol/L). Glutamic acid plus glutamine accounts for the most free amino acids in mature human milk and their sum represents 40% of total FAA. On the other hand, some amino acid derivatives such as citrulline, ethanolamine, ammonium, ornithine, ortophosphoserine, and phosphoethanolamine, not usually a part of protein, are determined and this fraction represented similar to 21% of the total FAA in mature human milk in the present study.