This paper deals mainly with the interfamily spheroplast fusion of Pasteurella multocida C 48-1 (Enr r,Chl s) and Escherichia coli O 78 (Chl r,Enr s), and the development of 2-combined attenuated strains. Protoplast fusion was performed by mixing protoplast suspension with 400 g/L PEG4 000 and newly formed Ca 3(PO 4) 2, and 16 strains hybrid cells with two drug-resistant markers were selected. To test the immunogenicity and safety of 2 fusion attenuated strains (F 4 and F 9), Guangdong yellow chicks were vaccinated separately with F 4 and F 9.The result showed that the F 4 strain was safe and with immunogenicity attributable to the two parent bacteria.