Abstract: 10dayold specific pathogen free (SPF) chickens were inoculated intratracheally with lowly virulent E. coli and /or mildly pathogenic avian influenza virus (MPAIV). The results were revealed as follows: (1) The mortality rate of chickens inoculated with E.coli and MPAIV simultaneously per bird was 33.3%. Chickens inoculated first with MPAIV and then E. coli at an interval of 48 hours experienced the mortality rate of 66.7%, whereas those inoculated with E. coli and then MPAIV at the same interval exhibited the mortality rate of 16.7%, the group infected with MPAIV alone experienced the mortality rate of 11.1% while the group infected with E. coli alone was found no death. These results suggested that MPAIV might longthen the periods that E. coli colonized in the trachea and lung of inoculated chickens, and it might facilitate E. coli invasion into the respiratory tract of the chickens resulting in more severe pathologic abnormalities. There was a pathogenic synergism between E. coli and MPAIV. (2) An indirect enzyme-linked immunosorbant assay for detection of antibodies to OMPs and an indirect hemagglutination assay for LPS of pathogenic E. coli from chicken origin were used respectively. The antibodies to OMPs and LPS of the birds inoculated first with MPAIV and then E. coli were lower than those of other groups. This results indicated that the synergistic pathopoiesis between E. coli and MPAIV seemed to be related to the immunosuppression exhibited in MPAIV inoculated birds.