Carcinoembryonic antigen (CEA), an oncofetal glycoprotein, is overexpressed in both colonic tumor tissue and in aberrant crypt foci. The 3' untranslated region (UTR) of CEA, which has been postulated to play a role in CEA expression, contains an oligo-A region. Sequence instability at oligo-A regions occurs in colorectal cancer, and it has been suggested that these mutations may alter expression of genes involved in the development of colonic tumors. We demonstrate that the oligo-A region of the CEA 3' UTR does in fact exhibit sequence instability. The significance of this instability, and of this region of the CEA 3' UTR in general, to CEA expression was analyzed in the HT29 cell line, colonic tumor cells which overexpress CEA upon induction with sodium butyrate. CEA promoter constructs, containing sense or antisense versions of the normal or mutant CEA 3' UTR region, were transfected into uninduced and induced HT29 cells. Although we could demonstrate that activity of the exogenous CEA promoter was markedly elevated in cells induced to differentiate, the data demonstrate no sequence or orientation-specific effect of the 3' UTR of CEA on expression from a CEA promoter. In addition, instability of the 3' UTR of CEA could be demonstrated in LoVo cells. However, there is no marked over- or underexpression of mutated 3' UTR CEA sequences in this cell line. We conclude that the oligo-A-containing region of the 3' UTR of CEA (204 bp beginning at nucleotide 24229) does not play a substantial role in the regulation of CEA expression.