Purified mouse immunoglobulins with specific activity against Mycoplasma pulmonis were examined for their ability to protect mice against intranasal (i.n.) infection with this organism. The survival of mycoplasmas in the respiratory tract of mice inoculated i.n. with M. pulmonis mixed with either IgG1, IgGa, IgG2b or IgA antibodies was less than that seen in control animals inoculated with mycoplasmas mixed with normal mouse serum. The ways in which the immunoglobulin fractions could have mediated resistance were examined in vitro. IgG1, IgG2a and IgG2b antibody fractions killed M. pulmonis in the presence of guinea-pig complement, and also promoted the phagocytosis of M. pulmonis by mouse lung macrophages. In contrast, IgA antibody did not appear to be either mycoplasmacidal or opsonic.