Clara-cell protein (CC16), the predominant protein secreted by bronchiolar Clara cells, increasingly appears to protect the respiratory tract against oxidative stress and inflammation. The aim of this study was to test in inbred strains of mice whether the lung susceptibility to O3 correlates with the transepithelial leakage of CC16, with the mRNA and protein levels of CC16, and possibly with specific isoforms of the protein in the respiratory tract. Five strains of mouse with increasing sensitivity to O3 (C3H, AKR, SJL, CBA, and C57Bl) were exposed to 1.8 ppm O3 for 3 h and examined at 0 and 6 h postexposure. The most sensitive (C57Bl) and resistant (C3H) mice were also continuously exposed to 0.11 ppm O3 for up to 3 days. Lung injury was evaluated by measuring in bronchoalveolar lavage fluid (BALF) the levels of total protein, albumin, lactate dehydrogenase (LDH), and inflammatory cells. The patterns of proteins in BALF were also analyzed by two-dimensional electrophoresis (2-DE). Exposure to 1.8 or 0.11 ppm O3 caused a transient elevation of CC16 in serum that was maximal immediately after exposure and closely correlated with the extent of lung injury evaluated by BALF markers. The epithelial damage assessed on the basis of serum CC16 or BALF markers showed an inverse relation with the preexposure levels of CC16 in BALF. Since preexposure levels of CC16 mRNA were similar between the strains and since lung epithelium damage was also negatively correlated with preexposure levels of albumin in BALF, these findings identify basal lung epithelium permeability as a determinant of susceptibility to O3. The 2-DE mapping of proteins in BALF of these two strains revealed the existence of two distinct isoforms of CC16 with pI values of 4.9 and 5.2. The most acidic form was significantly less concentrated in the C57Bl strain, the most sensitive to O3, a difference that might be related to the higher permeability of the lung epithelium or to some post-transcriptional variations. In conclusion, these results suggest that the permeability of the lung epithelial barrier may be an important determinant of the lung susceptibility to O3, controlling the intrapulmonary levels of CC16 and possibly of other antioxidant/inflammatory proteins.