The role of a-haemolysin (AH) in the resistance of Escherichia coli strains, isolated from patients with urinary tract infection, to the bactericidal activity of human serum and to effect of these bacteria on polymorphonuclear leucocytes (PMNL) morphOlogy was investigated in vitro. We determined that 64% of a-haemolytic E. coli was resistant to bactericidal activity of serum, but resistance rate on non-haemolytic E. coli was48%. The effect of bacteria on morphology of PMNL was observed; there was a significant difference between a-haemolytic E. coli and non-haemolytic E. coli (p