Variations of lactose and oligosaccharides in human milk were studied over a 3-month lactation period by dialysis, chromatography on Bio-gel P-2 column, paper chroma-tography, colorimetric analysis and gas chromatography. Milk samples were collected from donors of various blood and secretor types. An increase in the concentration of lactose was noted: 59.25 ± 1.61 to 72.17 ± 1.35 g/l for the group of secretor A secretor LeaLeb individuals, and from 62.25 ± 1.35 to 73.15 ± 3.45 g/l for the group of secretor H secretor Lea Leb individuals. A decrease in the concentration of oligosaccharides was also found: from 16.71 ± 0.99 to 7.90 ± 1.29 g/l and from 18.51 ± 0.74 to 7.33 ± 0.65 g/l, respectively, for these same groups. In secretor H nonsecretor Lewis individuals, the concentration of lactose increased from 67.97 ± 3.09 to 77.42 ± 1.88 g/l, and the concentration of oligosaccharides decreased from 13.27 ± 0.40 to 3.47 ± 0.33 g/l. Analysis of oligosaccharide composition by chromatography showed that variations are in relation to the secretor Lewis type. All oligosaccharides decreased equally throughout the lactation period, regardless of the blood group. The decrease was more rapid, however, in the nonsecretor Lewis individuals. In these individuals, the absence of certain oligosaccharides is not compensated by an overproduction of the other oligosaccharides present or by biosynthesis of structurally different oligosaccharides. The origin of these oligosaccharides i