The authors studied the species-specific and quantitative composition of the large intestinal microbiocenosis on exposures to various factors of endo- and exogenous etiologies: the presence of a pathological process and its specific features, dietary trace element composition, unfavorable environmental (chemical) factors, as well as the impact of coexistence in the same family and the factors of family variability (the individual genotype of the macro-organism). The microenvironment of non spore-forming anaerobes that colonize the large bowel was found to be influenced by a number of various factors, both exo- and endogenous. The magnitude of these changes is associated with the intensity and specificity of an influencing factor.