Class-specific antibodies were measured by a solid-phase microradioimmunoassay in the sera and lung washings of mice after intranasal or intravenous inoculation with live Mycoplasma pulmonis and after systemic, intranasal, or combined vaccination with Formalin-inactivated mycoplasmas. After intranasal or intravenous inoculation with live organisms, serum antibodies were first detected in immunoglobulin M (IgM) followed by IgG2, IgG1, and IgA classes, but significant levels of IgA developed only in those mice inoculated intranasally. The appearance of antibodies in lung washings was later than in serum, but again these were predominantly IgG2 and IgG1. After inoculation with killed organisms, serum antibodies were predominantly IgG1, although IgG2, IgM, and, in intranasally vaccinated mice, IgA were also present. Only IgG1 was detected in lung washings from mice vaccinated systemically, but IgA and IgG2 were present in addition in animals vaccinated intranasally. Immunofluorescence studies indicated that some antibody in lung washings from the latter group of animals was produced locally. A comparison of the levels of various class-specific antibodies and resistance to intranasal challenge suggested that local antibody of any immunoglobulin class is capable of mediating resistance in the lungs to M. pulmonis infection.