The aim of the study was to determine the concentrations of macronutrients and the mineral and trace element composition in maternal milk of Japanese women. We collected human milk samples from mothers living throughout Japan from December 1998 to September 1999, and defined as group A the 1197 samples among them that met the following conditions: breast milk of mothers who were under 40 years old, not in the habit of smoking and/or using vitamin supplements, and whose babies showed no symptoms of atopy and whose birth weights were 2.5 kg or more. We then analyzed their contents individually. We also analyzed the amino acid and free amino acid composition of the breast milk of pooled samples from various lactation stages. Large differences were found to exist among the contents of individual human milk samples. The mean contents of each component were as follows: energy, 66.3+/-13.3 kcal/100 mL; solid matter, 12.46+/-1.56 g/100 mL; ash, 0.19+/-0.06 g/100 mL; total nitrogen, 0.19+/-0.04 g/100 mL; lipids, 3.46+/-1.49 g/100 mL; carbohydrates, 7.58+/-0.77 g/100 mL; lactose, 6.44+/-0.49 g/100 mL; pH, 6.5+/-0.3; osmotic pressure, 299+/-14 mOsm/kg.H2O; chloride, 35.9+/-16.2 mg/100 mL; sodium, 13.5+/-8.7 mg/100 mL; magnesium, 2.7+/-0.9 mg/100 mL; phosphorus, 15.0+/-3.8 mg/100 mL; potassium, 47.0+/-12.1 mg/100 mL; calcium, 25.0+/-7.1 mg/100 mL; chromium, 5.9+/-4.7 microg/100 mL; manganese, 1.1+/-2.3 microg/100mL; iron, 119+/-251 microg/100 mL; copper, 35+/-21 microg/100 mL; zinc, 145+/-135 microg/100 mL; and selenium, 1.7+/-0.6 microg/100 mL. The content of each component varied greatly as the duration of lactation increased. In conclusion, it appears to be necessary to evaluate individual differences of human milk in order to perform valid research regarding infant formula.