Abstract: OBJECTIVE To study the levels of complement activation and immune factors in neonatal rats infected with Mycoplasma pneumoniae.METHODS Totally 42 neonatal SD rats were randomly selected so as to prepare acute lung injury models and were divided into the blank group and the model groups（group M1, M2, M3, M4, M5）. The health status of the model groups was observed, the counts of inflammatory cells in peripheral blood were determined, the pathological status was observed, and the expression levels of lung tissue-related immune factors and peripheral blood complement proteins（C3 a, C4 a, C5 a） were detected by enzyme-linked immunosorbent assay.RESULTS The mental state of the rats in the model groups declined, and the body weights were decreased（P＜0.05）, the rats had obvious pathological damage of lung tissues and infiltration of inflammatory cells. The counts of inflammatory cells in peripheral blood were significantly greater in the model groups than in the blank group, and the titers of Mycoplasma inclusion bodies were significantly increased with time（P＜0.05）. The expression levels of interleukin（IL）-6, IL-12, IL-17 A, transforming growth factor β1（TGF-β1） and tumor necrosis factor α（TNF-α） in lung tissues were significantly higher in the model groups than in the blank group and the control group（P＜0.05）. The activation levels of peripheral blood C3 a, C4 a and C5 a complements of the model groups were significantly higher than those of the blank group and the control group（P＜0.05）.CONCLUSION There is association between the pathological changes of lung tissues of the neonatal rats infected with M.pneumoniae and the expression levels of IL-6, IL-12, IL-17 A, TGF-β1 and TNF-α in lung tissues as well as the activation levels of complements. With the extension of time, the degree of damage of lung tissue is affected by the activation level of complements.