The current incidence of E. coli strains in healthy humans capable of producing the inhibitive exoproducts such as temperate bacteriophages, corpuscular or HMW (high molecular weight) and proteinaceous or LMW (low molecular weight) colicins and siderophores was studied. 53 Escherichia coli strains were collected from the colons of 53 healthy human volunteers in Brno, Czech Republic. The strains were then tested for spontaneous and induced production of inhibitive exoproducts in a cross-test against each other. Of the strains tested, 39.6% produced bacteriophages, 43.4% of strains produced from one to several LMW colicins. Only 5.7% of strains formed HMW colicins. And 15.1% of strains (eight strains) produced exocellular siderophores different from enterochelin. Of the eight strains, seven strains formed aerobactin and one strain formed an untyped siderophore. E. coli strains differ greatly in the incidence of colicinogeny and lysogeny from its closest systemic relatives in the genus Escherichia and therefore should not be regarded as a model bacterium in this respect.