Seventeen strains of E. coli, isolated from chickens with colisepticaemia, were studied with respect to their pathogenic characteristics including: serum resistance, toxin production, pathogenicity for one-day-old chicks, colicin production, adherence to and invasiveness of HeLa cells, plasmid DNA profile and SDS-PAGE electrophoresis of membrane proteins, as well as electron microscope studies and hemagglutination tests for fimbriae. We concluded that the adherence to and the invasiveness of HeLa cells were not related to the pathogenicity of these strains for chickens. Plasmid profiles were not related to the bactericidal activity of the serum. Toxin production was correlated to the highest levels of pathogenicity. Some of the strains had mannose-resistant fimbriae. SDS-PAGE of membrane proteins of all the strains which were either not pathogenic or which had a very high LD(50) lacked two major protein subunits of 40.7 kDa and 28.8 kDa found only in pathogenic strains.41416717586