In several groups of probands the influence of nutrition on the gut flora has been examined. In old people there is a striking decrease in the numbers of Bifidobacteria whereas those of Clostridium perfringens strongly increase and those of Enterobacteriaceae and Lactobacilli can be found somewhat higher than in the compared group. In a group of longevitial old aged men nourished with a fibre-rich diet these alterations are found less striking. Changing from a mixed Japanese diet to a "western diet" a tendentious decrease in the total number of bacteria and in the numbers of Bifidobacteria, Eubacteria, and in moderate, in the numbers of Streptococci, too, could be determined. Simultaneously, a considerable development of nitrosamines occurred. In C3H/Hemice a glucomannan containing (fibre-rich) diet caused a substantial increase in the evidence frequency of Bifidobacterium and in the numbers of Enterobacteriaceae. Parallel with these results incidence and average number of noduli of manifested hepatic carcinoma decreased remarkably.