In the immunoblot analysis, sera from patients infected with Mycoplasma pneumoniae reacted with the 168kDa (P1) and the 85kDa proteins of virulent strain FH-P24 and P24-S1 mutant strain but not with the 85kDa protein of P24-S11. Sera of hamsters and BALB/c mice, which had been immunized with live vaccines, were tested. In FH-P24 immunized animals, 100% or 80%, and in P24-S1, 40% of hamsters and 60% of BALB/c mice, developed antibodies against the 85kDa protein, but antibodies were not detected in sera of P24-S11 immunized animals. The correlation between the development of antibodies to 85kDa protein in the sera of vaccinated animals and the effects of protection by living vaccines were suggested.