Immunoglobulins A, M and G were measured in the milk of 20 nursing mothers at the beginning of lactation and simultaneously in the faeces of their children. The IgA level in the human milk was very high especially during the first week of lactation. After the start of breast-feeding IgA rapidly increased in the faeces, whereas IgG and IgM concentrations were consistently very low. In 74 normal full-term bottle-fed infants IgA appeared in the faeces at the age of 3-4 weeks but the level was significantly lower than the IgA level in breast-fed infants at the same age. Secretory IgA of human milk is stable and resistant to gastrointestinal juices and enzymes, thus giving passive immunological protection to the digestive tract of the newborn infant.