A newly established mouse strain, NIPS, which is more sensitive to Mycoplasma pulmonis than ICR, ddY and other mouse strains was examined for its susceptibility to Mycoplasma pneumoniae. In experimental infections with M. pneumaniac, it was observed that M. pneumoniae attached to tracheas of MPS mice, and M. pneumoniae cells were isolated from tracheas and lungs of MPS mice even after four weeks of infection, while no mycoplasmas were isolated from ICR and ddY mice after one week of infection. Specific antibodies against M. pneumoniae were also observed by the Western blotting in the sera of MPS mice infected with M. pneumoniae. Although any lung lesions could not be observed in this work, this newly established mouse strain NIPS may be useful for experiments of M. pneumoniae infection, especially fbr the analysis of strain differences in susceptibility to M. pneumoniae infection.Mycoplasma pneumoniae is an agent which causes acute respiratory infection including primary atypical pneumonia in human (2-4, 9, 14). As an animal model for human disease with this agent, in general, Syrian hamster is used (2, 5, 6). Syrian hamster inoculated intranasally with M. pneumoniae can develop a respiratory infection which is characterized by peribronchial infiltration similar to lesions seen in sections of human lung infected with this agent (1). It was said that mice are not sensitive to M. pneumoniae infection, but Kanamori and Katsura et al (7,8) showed that suckling ICR mice were sensitive to M. pneumoniae infection. They demonstrated that M. pneumoniae cells were isolated from the lung of suckling ICR mice exposed to Al. pneumoniae for 2 weeks after infection, and specific antibodies to M. pneumoniae were raised in the sera of mice.Previously we showed remarkable strain differences in the development of gross pneumonic lesions in the mice infected with Al. pulmonis (11). Among ddY, ICR, NI H, CF#1 and C3H/He strains, ICR mice were most sensitive to Al. pulmonis infection. From those ICR mice, a more sensitive strain to M. pulmonis which can produce pneumonia in more than 70% of the mice infected with M. pulmonis was established and is referred to as MPS strain (unpublished). Then, we investigated the susceptibility of MPS mice to M. pneumoniae infection. M. pulmonis-sensitive ICR and low-sensitive ddY mice were also used as controls. Four-week-old (both 247