In the mouse burn model of Stieritz and Holder [19], a three-component vaccine consisting of elastase toxoid, protease toxoid and the common antigen (OEP) of P. aeruginosa was found significantly effective on protection against subcutaneous injection with strain IFO 3455, but not with strains M2 or PF 2243. Investigating the differences, it was clarified that somatic antigens of strains M2 and PF 2243 did not contain serological antigens cross-reacting with OEP. Accordingly, the protective effects of the somatic antigen of strains M2 were investigated with the result that both formalin and heat-killed antigens demonstrated significant protection against challenge with strain M2 in the mouse burn model. As strains PF 2243 and M2 were found to belong to the same serotype 16 of Homma's schema [23], it is presumed that somatic antigens of serotype 16 are devoid of OEP. At the same time, it is suggested that the somatic component of P. aeruginosa is essential to the effects of the multi-component vaccines on protective immunity.