The state of the microflora of the large intestine in 18 patients with infectious endocarditis (IE) was studied. The study revealed that the existing specific features of the microbial picture of the large intestine were caused by changes in the content of some representatives of the indigenous group of microbes, as well as in the concentration and occurrence of transitory microbial species. Changes in the character of the microbial picture were found to depend on the activity of IE, as well as on the dynamics of the disease. In the course of this study no correlations between changes in the microbial picture of the intestine and the sex of the patients, as well as between the fact of the infection being primary or not and the presence of different clinical symptoms and syndromes of IE, were established. On the basis of the data thus obtained the most informative diagnostic signs reflecting the general character of changes typical of this disease were selected.