Abstract: According to Haemophilus parasuis which was isolated from the department,the strain of poison strong,stable growth and virulence stability was selected as the original strain. The original strain was induced to cause a mutation by chemical agents,then after toxicity test,significantly less virulent strains were selected. The safety trials found that these attenuated strains had a high security, and layed the foundation for the development of attenuated vaccine.

Key words: Haemophilus parasuis; chemically induced; attenuated strains; attenuated vaccine