A total of 981 human Escherichia coli strains (including 632 strains isolated between 1979 and 1983 and 349 strains isolated in 1987) was examined for aerobactin production by biological qualitative test. Aerobactin positivity was found in 55.1% and 47.3%, respectively, in the two groups of strains, while enterochelin was produced nearly by 100% of the strains. Aerobactin production was significantly more frequent than the average among blood and CSF strains samples and serogroup O2 and O6 strains. Aerobactin was more frequent among isolates with K1 or K5 antigens and producing haemolysin and mannose-resistant haemagglutination than among the ones lacking these virulence factors. A strict correlation was found between the pathogenicity in mouse following intraperitoneal infection and the frequency of aerobactin production. The distribution of the LD50 values of the aerobactin positive strains was shifted towards the lower values comparing to the aerobactin negative ones, proving statistically the effect of aerobactin in the increase of pathogenicity.