In this study we determined whether the alveolar fluid content of a specific epithelial type I cell protein, rTI40, can be used as a biochemical marker for lung injury. A model of alveolar epithelial injury was developed by instilling Pseudomonas aeruginosa bacteria (PA103) into the airspaces of anesthetized, ventilated rats. After 6 h, the alveolar fluid content of rTI40 from PA103-treated rats was increased over 80-fold in comparison to alveolar fluid from control rats (P < 0.05). This increase in rTI40 correlated with both morphological evidence of injury to alveolar epithelial type I cells and increased permeability of the alveolar epithelium to protein tracers. In contrast, the lactate dehydrogenase activity of alveolar fluid from PA103-treated rats was elevated only threefold over control values at 6 b (P < 0.05). In a second study using a less injurious strain of P. aeruginosa (PA103 exsA::Ω), the alveolar fluid content of rTI40 was the same as control values. These findings indicate that the alveolar fluid content of a type I cell-specific protein can be used as a sensitive and specific biochemical marker of type I cell injury.