With the analysis of more than 550 human milk samples we showed that triglycerides (accounting for 96-99% of the total lipids) increased from 2 to 3.5 g/100 ml mainly during the first week postpartum and remained constant thereafter. In contrast, both cholesterol and vitamin E concentrations decreased from 35 to 20 mg/100 ml and from 1.7 to 0.30 mg/100 ml, respectively. The phospholipids remained constant (40 mg/100 ml). Also the fatty acid composition of total lipids changed. Mid chain fatty acids (C10, C12, C14) increased, whereas the long chain polyunsaturated fatty acids decreased. The P/S-ratio of 0.32 remained constant throughout lactation. The mean protein content of human milk decreased from approximately 2 g/100 ml at day 2 to approximately 1 g/100 ml at day 36 of lactation. The content of each individual amino acid decreased likewise. However, when we expressed the amino acid values in relation to the protein value--that is as g amino acid per g protein--some decreased, some remained constant and some increased indicating a changing protein pattern. From these data we computer-calculated a sharp decrease in IgA content, a moderate decrease for lactoferrin and constant values for casein and a-Lactalbumin. Throughout the whole period of investigation, non protein nitrogen accounted for about 20% of total nitrogen. Although the absolute values decreased according to total nitrogen, the sum of free amino acids, as well as the amount of taurine, remained constant. Lactose increased from about 4 g/100 ml to 6 g/100 ml during the first days of lactation. The calcium content increased from a mean value of 25 mg/100 ml at day 1 to 32 mg/100 ml at day 5 and remained constant at 30 mg/100 ml up to day 36. Phosphorus content increased from 10 mg/100 ml at day 1 to 17 mg/100 ml at day 8 and then continuously decreased to 13 mg/100 ml at day 36. The protein-bound part of the calcium remained constant during the period investigation, the fat-associated part increased from 11% to 26%. Protein-bound phosphorus decreased from 45% in colostral milk to 29% in transitional and 23% in mature milk.