ABSTRACT. As a b2 integrin family member, Mac-1 plays an important role in the inflammatory response. Inflammation and lung injury are closely associated, but the involvement of Mac-1 in the occurrence and development of such pathologies remains poorly understood. We aimed to investigate the relationship between Mac-1 deficiency and respiratory failure in Mac-1 knockout {Mac-1 -/-} mice, using C57BL/6J mice as a control. The newborn survival rate of Mac-1 -/-mice was calculated, and mouse lung tissue was treated with hematoxylin and eosin and subjected to immunofluorescent staining. Moreover, western blotting and immunohistochemistry were used to detect the expression of molecules specific to type I and type II alveolar epithelial cells, as well as alveolar surfactant proteins secreted by the latter. Survival of Mac-1 -/-pups was significantly lower than that of newborn C57BL/6J mice. In a float test, lung tissues from C57BL/6J mice were buoyant, whereas those of Mac-1 -/-mice were not. Compared with C57BL/6J mice, expression of proSP-C {specific to type II alveolar epithelial cells} and alveolar surfactant proteins in Mac-1 -/-mice was not significantly different, implying that type II cell function was unaltered. However, western blotting revealed expression of T1α, Aqp5, and Snx5 {type I alveolar epithelial cell markers} in Mac-1 -/-mice to be significantly decreased {P < 0.05}. In conclusion, Mac-1 may play an important role in respiratory failure. Its absence leads to this condition not by influencing type II alveolar epithelial cells or their secreted surfactant proteins, but rather by reducing type I alveolar cell numbers.