Carbonic anhydrase 1 (CA1) catalyses the reversible hydration of CO2 and is important for cellular diffusion of CO2, ion transport and pH regulation. The gene encoding CA1 (CA1) has two promoters. In adult colon epithelia the proximal promoter determines high levels of expression and the distal erythroid promoter is repressed. RNA in situ hybridisation shows that CA1 mRNA is abundant in differentiating cells of the colonic crypt as they migrate to the luminal surface, but is not present at the base of the crypts and levels are low on the luminal surface. It is likely that CA1 gene expression in these cells is regulated by differential transcription and/or mRNA stability. In contrast CA1 protein is localised predominantly on the luminal surface. Since CA1 mRNA and protein do not exactly co-localise it can be inferred that CA1 expression is also subject to post-transcriptional control. CA1 mRNA is significantly reduced in colon carcinoma and in adenomas from familial adenomatous polyposis patients. Loss of CA1 expression is associated with the disappearance of differentiated epithelial cells. Out of twelve colon carcinoma cell lines three, LIM1215, LIM1899 and HT115, expressed CA1 and nine did not. This variation in expression may also be associated with cell type differentiation.