We report here the identification and characterization of a novel gene, T1 alpha, expressed in high abundance in adult rat lung, fetal lung, and early fetal brain. T1 alpha was identified by a monoclonal antibody previously shown to be specific for an antigen expressed by alveolar epithelial type I cells. The cDNA for T1 alpha is 1.85 kb and identifies a single mRNA species of the same size on Northern blots of adult rat lung. The longest open reading frame of the cDNA is 498 bases which would encode a protein of approximately 18 kDa. The protein has a putative membrane spanning domain near the C-terminus but lacks consensus sequences for N-glycosylation. Northern blots and RT-PCR show high expression of T1 alpha in adult lung, with marginally detectable expression in adult brain, intestine, and kidney. RT-PCR analysis shows expression of T1 alpha in freshly isolated type I cells (50-60% purity) but not in highly purified type II cells or other lung cells. We believe therefore that T1 alpha is primarily if not uniquely expressed in alveolar type I cells in the adult rat. Polyclonal antisera against a 16-amino-acid peptide identified in the deduced sequence reacts with the apical membranes of adult type I cells in lung tissue sections but does not label other cell types. The above antiserum as well as the original monoclonal antibody recognize a single approximately 18-kDa protein derived from bacterial expression of a construct containing the T1 alpha open reading frame. By RT-PCR T1 alpha is detected in rat lung from Day 13.5 onward, but is detected by in situ hybridization earlier in lung, brain and neural derivatives, and foregut. Expression is down-regulated in all but lung tissues as development proceeds.