Bacteriological studies of uncontaminated upper jejunal fluid were performed in 85 normal subjects. Fifty-three per cent of the samples were sterile (less than 10(1) CFU/ml). In 10% of the cases the total number of microorganisms exceeded 10(5) CFU/ml. The main groups of microorganisms isolated were Streptococcus sp ('Viridans group'), Lactobacillus sp., Veillonella parvula, Actinomyces sp., Haemophilus sp., Corynebacterium sp., and Candida albicans, each found in more than 10% of the subjects. Only the Streptococcus sp. exceeded 10(5) CFU/ml, and enterobacteria were found in 5% of the subjects, the number not exceeding 10(3) CFU/ml. No other typical members of the lower gastrointestinal tract were isolated. The number of subjects harbouring bacteria and the distribution of bacterial species were the same in both sexes and in different age groups.