The role of type-1 fimbriae in the pathogenesis of chronic pyelonephritis was studied for two Escherichia coli strains. Although both strains produced a similar total oxidative burst of chemiluminescence in macrophages from uninfected mice, the extracellular oxidative burst was greater with the non-fimbriate mutant E. coli BH-5 than its type-1 fimbriate parent E. coli 31-B. Moreover, macrophages from mice infected with the non-fimbriate mutant gave a much greater oxidative burst when stimulated with latex particles than that given by macrophages from mice infected with the type-1 fimbriate parent. These results correlated with the degree of renal inflammation and scarring as measured by malondialdehyde formation. Hence, the role of type-1 fimbriae in the pathogenesis of chronic UTI although documented does not appear to be significant.