This study was designed to establish whether the starting time of breast-feeding (BF) may have affected gut closure for macromolecules during the earliest postnatal period. An increase of serum IgA concentrations was taken as the index of gut permeability to macromolecules. In 14 neonates, BF started 1-6 h after birth and continued every 3 h during the next 38-48 h; in 19 neonates, it started 12-15 h after birth, continuing in the following 34-46 h; and in 20 neonates, BF started at the age of 24-29 h and continued during the next 18-36 h. Serum IgA concentrations were measured before and after the period of BF using the method of single radial immunodiffusion on commercial plates. In neonates in which BF started earlier, serum IgA concentrations fell significantly (p less than 0.001) until the 3rd day of life. This suggested an early gut closure, caused either by some colostral factor(s) or, perhaps, by some other mechanism which prevented further macromolecular absorption. In 11 of 20 neonates in which BF started after 24 h of life, even though in this group of babies, it was of the shortest duration, serum IgA concentrations increased significantly (p less than 0.001), suggesting that if BF is postponed, a spontaneous gut closure may not take place within the first 30 h of life.