A study of 41 strains ofShigella, Escherichia coli, andSalmonellawas performed. The presence of high‐molecular‐weight plasmids, invasiveness in HEp‐2 cells and the ability to produce a positive Serény test were tested. Five of the seven strains ofSalmonella typhimuriumharboured a 62 Md plasmid and invaded HEp‐2 cells. Two strains ofS. typhimuriumand aS. paratyphi‐Bstrain lacked plasmids but were still invasive. Among the 27 strains ofShigellaand enteroinvasiveE. coli(EIEC) 25 strains harboured a high molecular weight plasmid. 27 of theShigella/EIEC strains invaded HEp‐2 cells and 25 produced a positive Serény test. One strain ofShigella sonneiwas invasive in HEp‐2 cells and gave a positive Serény test, but plasmids were not demonstrated. Of the eight non‐enteropathogenicE. coli5 strains harboured plasmids of 100 to 140 Md size; only one of the strains invaded HEp‐2 cells; none of the strains produced a positive Serény test. The study shows that tests for pathogenicity (Serény test, HEp‐2 cell test) are usually positive in theShigellaand EIEC group of bacteria; these bacteria also usually carry a high molecular weight plasmid. However, among non‐enteropathogenic bacteria plasmids of 100 to 140 Md size can be observed without any correlation to invasive properties. Genetic information from gene loci located to plasmids and chromosomes is required to give a positive Serény test. The presence of high molecular weight plasmids does not seem to be neessary for expression ofin vitroin