An infection model in laboratory mice for studying the bacterium (proposed name Haemophilus equigenitalis) causing contagious equine metritis is described. Small porous chambers were implanted subcutaneously into mice and after 1 to 3 weeks were inoculated with H equigenitalis. Infections that persisted for > 30 days were established by direct transfer of infective chamber fluid or by injection of laboratory-grown cultures. Immunization of mice with formaldehyde-treated cells induced significant, strain-related immunity to infection and did not appear to require complement as a protection mediator. Substantial differences in staining characteristics and cell morphology were observed between bacteria grown in mouse chambers and those grown on a laboratory medium. These differences were more apparent in smears of the organism stained by a modified Gimenez method than in smears stained by the conventional Gram stain. This murine model may be a convenient and inexpensive method for studying the immunobiology of this newly discovered animal pathogen.