We have investigated a mechanism of the regulation of mucin core polypeptide (MUC1) gene expression, which is induced by a soluble stimulatory factor, in KM12C human colon carcinoma cells. Conditioned media from normal human colon tissues elevated the level of expression of MUC1 mRNA. Transcriptional activation of the MUC1 gene was analyzed by transient expression of MUC1-CAT reporter plasmids containing the 5'-flanking sequence of the MUC1 gene fused to the bacterial chloramphenicol acetyltransferase (CAT) gene. A region between base pairs -531 and -520 of the 5'-flanking sequence of the MUC1 gene was sufficient for the induction of CAT activity by normal colon conditioned medium (NCCM). Mutagenesis of 3 base pairs within the region corresponding to sequence -531 to -517 from ACAGGGAGCGGTTAG to ACAGGGAGATTTTAG substantially decreased the induction of CAT activity by NCCM. Nuclear extracts from untreated or NCCM-treated KM12C cells were tested for their interaction with 32P-labeled oligonucleotides corresponding to this sequence. A specifically retarded band was identified after electrophoretic analysis. The quantity or mobility of this band was not changed by NCCM treatment. When an oligonucleotide with three point mutations was used as a competitor, the retarded band remained at the same position. This element (positions -531 to -520), which we call the responsive mucin element, does not contain any sequence that corresponds to previously described cis-acting elements. A protein component complexed with this sequence was identified with a molecular mass of approximately 70 kDa by SDS-polyacrylamide gel electrophoresis.