Escherichia coliO157 strains belonging to a distinct lineage and expressing different O-antigen (Oag) lengths were isolated. Although the function ofwzzinE. colihas not been adequately investigated, this gene is known to be associated with regulation of Oag length. UsingE. coliO157:H7 ATCC43888 (wild-type), severalwzzmutants ofE. coliO157, including awzzdeletion mutant, were generated and the relationship between the length of Oag modulated by thewzzgene and sensitivities to serum complement investigated. SDS–PAGE, immunoblot analyses and sensitivity tests to human serum complement were performed on these strains. The lengths of the O157-antigen could be modulated by thewzzgene mutations and were classified into long, intermediate and short groups. The short chain mutant was more serum sensitive than the wild-type strain and the otherwzzmutants (P< 0.001). In conclusion, Oag chain length modulated by thewzzgene inE. coliO157 influences its sensitivities to serum complement. The present findings suggest thatE. coliO157 strains with intermediate or long length Oag chains might show greater resistance to serum complement than those with short chains.