We report here the identification and characterization of a novel gene, T1α, expressed in high abundance in adult rat lung, fetal lung, and early fetal brain. T1α was identified by a monoclonal antibody previously shown to be specific for an antigen expressed by alveolar epithelial type I cells. The cDNA for T1α 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 ∼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 T1α in adult lung, with marginally detectable expression in adult brain, intestine, and kidney. RT-PCR analysis shows expression of T1α 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 T1α 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 ∼18-kDa protein derived from bacterial expression of a construct containing the T1α open reading frame. By RT-PCR T1α 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.