In a group of 50 strains of E. coli isolated from infants hospitalized on account of diarrhoea the authors investigated, based on the phenotypic manifestation and in a selected number of strains also genotypic factors of virulence: fimbrial adhesins, aerobactin, haemolysins, colicins, toxic and lethal action in mice, seroresistence and resistance against tetracycline, streptomycin, chloramphenicol, kanamycin, ampicillin and sulfamethoxidine. All strains had one to eight factors of virulence. The presence of plasmid DNA was confirmed in 14 of 16 tested strains. From this the authors conclude that the investigated virulence factors were in the majority coded plasmids.