Analysis of CNBr fragments and other peptides from human liver cytoplasmic aldehyde dehydrogenase enabled determination of the complete primary structure of this protein. The monomer has an acylated amino terminus and is composed of 500 amino acid residues, including 11 cysteine residues. No evidence of any microheterogeneity was obtained, supporting the concept that the enzyme is a homotetramer. The disulfiram-sensitive thiol in the protein, earlier identified through its reaction with iodoacetamide, is contributed by a cysteine residue at position 302, while the cysteine which in horse liver mitochondrial aldehyde dehydrogenase is reactive with coenzyme analogs appears to correspond to either Cys-455 or Cys-463. Analysis of glycine distribution and prediction of secondary structures to localize βαβ regions typical for coenzyme-binding are not fully unambiguous, but suggest a short region around position 245 as a likely segment for this function. In this region, sequence similarities to parts of a bacterial aspartate-β-semialdehyde dehydrogenase and a mammalian alcohol dehydrogenase were noted. Otherwise, no extensive similarities were detected in comparisons with characterized mammalian enzymes of similar activity or subunit size as aldehyde dehydrogenase (glyceraldehyde-3-phosphate dehydrogenase and glutamate dehydrogenase, respectively).