Bacteriological studies of uncontaminated upper jejunal fluid were performed in 51 Danish children without gastrointestinal disorders. Thirty-seven percent of the samples were sterile [less than 10(1) colony-forming units (CFU)/ml]. In 25% of the cases, the total number of microorganisms exceeded 10(5) CFU/ml. The microorganisms isolated most frequently were: Streptococcus, sp. ("Viridans group"), Veillonella parvula, Hemophilus parainfluenzae, Lactobacillus, sp., Corynebacterium, sp., Actinomyces, sp., Bacteroides, sp., and Hemophilus influenzae, each found in more than 10% of the children. The upper range for the number of microorganisms isolated exceeded 10(5) CFU/ml for most of the species isolated of which the vast majority belonged to an "oral type" of flora. Enterococci were isolated in small numbers in three children, and Enterobacteriaceae were not found.