Mycoplasma pulmonis and Mycoplasma arthritidis were differentially identified using PCR-restriction fragment length polymorphism (RFLP).M. pulmonis is known to play a major etiological role in murine respiratory diseases in long-term rodent studies [3]. In addition, this organism produces genital infections resulting in reduced birth rates [3], and Cesarean derivation and barrier maintenance have little effect on prevention of in utero infection of M. pulmonis. A serological test is commercially used for diagnosis of M. pulmonis, however antigenic cross-reactivity between M. pulmonis and M. arthritidis interferes with interpretation of the serological test results. Accordingly, serological differentiation of M. pulmonis from M. arthritidis is needed because M. arthritidis is naturally non-pathogenic and does not interfere with laboratory rodent research, though it is arthritogenic in mice after intravenous inoculation [9]. Seoul, South Korea The objective of the present study was to develop a method of differential diagnosis of M. pulmonis and M. arthritidis.Thirty mice and rats showing dyspnea, chattering signs and seropositivity were collected from several conventional facilities. The serological test for M. pulmonis was performed using Murine Immunocomb™ (Charles River, U.S.A). The animals were euthanized by cervical dislocation, and the skin over the trachea was grasped with forceps and excised with scissors. By excising the sternohyoid muscle the trachea was exposed, washed with phosphate buffered saline (pH 7.2) and the tracheal fluid was collected. The tracheal wash was cultured on a PPLO agar plate supplemented with yeast extract (0.1 mg/ml), horse serum (20%), penicillin (3.5 mg/ml), and