Abstract Studies of aerobactin excretion, production of antimicrobial agents, haemolysin production, serum resistance and presence of plasmids were made in 157 clinical isolates of Escherichia coli, 96 from blood and 61 from faeces. Only serum resistance and the presence of a small number of plasmids (< 4 per cell), appeared more frequently in strains from blood than in faecal isolates. It was noteworthy that combination of aerobactin, antimicrobial agents and haemolysin production was more frequent in blood isolates than in strains from faeces. A substantial number of strains from blood presented all 5 virulence factors (9.4%) while none of the faecal strains exhibited this phenotype. Definition of the systemic invasivity of E. coli as a multi-factorial phenomenon is suggested.