Strains of E. coli isolated from the urine of children with pyelonephritis (P) or asymptomatic bacteriuria (ABU) were compared with respect to serotype, amount of capsular antigen, opsonic requirements, sensitivity to serum and phagocytosis by neutrophils. The ABU-strains more often had the capacity to activate alternative complement pathway (p < 0.001), were better opsonized in fresh human serum (p < 0.01) and were more often serum sensitive (p < 0.001) than the P-strains. Neutrophil chemiluminescence during phagocytosis of the ABU- and P strains did not differ significantly. There was no correlation between presence of a particular O or K antigen and opsonic requirements. However, the P strains produced greater amount of capsular antigen than the ABU strains. The results indicate that ABU strains more often are sensitive to non-specific host defence mechanisms than P strains even when similar serotypes are tested. The differences may be relevant for further studies of bacterial virulence and host defence factors.