To determine whether c-kit and kit ligand (KL) mRNAs could be expressed in human epithelial tumors, reverse transcrip-tase-polymerase chain reaction and Northern blot analysis were performed. KL mRNA was shown to be expressed in a variety of epithelial tissues and cell lines. The expression of c-kit mRNA was then examined in hepatocellular and colon carcinoma cell lines. While hepatocellular carcinoma cell lines did not express c-kit mRNA as far as we could ascertain, 2 of 5 colon carcinoma cell lines showed the expression of both c-kit and KL mRNAs. Furthermore, the expression of c-kit in these cells was demonstrated at the protein level by flow cytometry. These data suggest that c-kit and KL may play an important role as an autocrine loop in the proliferation of some colon carcinoma cells.