The promoter activity of the upstream region of the rat small intestinal lactase-phlorizin hydrolase gene has been analysed by transfection in the human colon cancer cell line Caco-2. A 0.9 kb mRNA, corresponding to the CAT reporter gene, was synthesized from the transcription start site of the LPH gene. The rate of expression, determined by semi-quantitative RT-PCR, was very low, and depended on the length of the promoter fragment in front of the reporter gene. By immunocytology, we found that the low level of expression resulted from the low number of cells (about 1%) in which CAT was produced. The endogenous lactase was present in 10-20% of the cells in culture, and evidence is provided that most cells that expressed CAT did not co-express the endogenous lactase. We conclude from this study that the rat small intestinal LPH gene promoter is active in the human Caco-2 colon cancer cells. Hence Caco-2 cells constitute an in vitro model to analyse the basic molecular mechanisms involved in the gene transcription of intestinal digestive enzymes. Yet, the mosaic expression of the endogenous lactase and of the reporter gene under the control of the rat LPH gene promoter, suggests that Caco-2 cells may present specific regulatory mechanisms of expression of small intestinal enzymes, possibly in relation to their tumourous origin.