To evaluate the contribution of the subsets of T helper lymphocyte (Th) to the development of pulmonary lesion in mycoplasma pneumonia, we compared the pathological findings between Thl dominant mice (C57BL/6) and Th2 dominant mice (BALB/c) in experimental Mycoplasma pulmonis (M. pulmonis) pneumonia. Mice (ICR, C57BL/6, and BALB/c) were intranasally inoculated with 0.03 ml of a solution containing M. pulmonis (1×10⁸) colony forming units per ml. Another M. pulmonis inoculated ICR mice were treated with interleukin-2 (IL2; 4.8μEg/day), days 3-9, intracutaneously. All mice were sacrificed at day 14, and the lung specimens were examined. Peribronchial lymphocyte cuffing was more prominent in C57BL/6 mice than that of ICR mice, and intra-alveolar inflammatory-cell infiltration in BALB/c mice was more prominent than in ICR mice. Pathological patterns of the lung in IL-2 treated ICR mice were mimicking those of C57BL/6 mice. These results suggested that pathological patterns of mycoplasma pneumonia in mice might be altered by the imbalance of host T helper lymphocyte subset.