Mice inoculated intranasally with either a virulent or an avirulent strain of live Mycoplasma pulmonis were resistant to respiratory disease induced by a subsequent intranasal challenge with virulent organisms. Similarly, mice inoculated intravenously with the virulent strain were resistant to intranasal challenge with the same strain. In contrast, mice inoculated intravenously with avirulent M. pulmonis were not resistant to intranasal challenge with the virulent mycoplasma strain. Studies on mice inoculated intravenously with the two strains of M. pulmonis indicated that persistance of mycoplasmas in the respiratory tract may be important in inducing resistance to intranasal challenge with M. pulmonis. These observations, together with the lack of correlation between the level of serum antibodies and resistance to M. pulmonis-induced respiratory disease, suggested that local immune mechanisms were important in resistance. It is proposed that an effective vaccination schedule to protect mice against M. pulmonis-induced respiratory disease may be one that stimulates both systemic and local immune defenses. This suggestion is supported by the observation that systemic followed by local administration of inactivated M. pulmonis was more effective in inducing resistance in mice to intranasal challenge with live organisms than was systemic administration alone. In addition, mice inoculated solely by the intranasal route with inactivated mycoplasmas were resistant to M. pulmonis-induced respiratory disease. These studies indicate the importance of local defense mechanisms in the induction of resistance to M. pulmonis-induced respiratory disease in mice.