The promoter of the pig lactase-phlorizin hydrolase was cloned and showed to be functional in the human intestinal cell line Caco2. The proximal promoter was analyzed for binding of nuclear proteins from small intestine and liver. DNase I footprinting and electrophoretic mobility shift assays show, that an intestinal nuclear factor (NF-LPH1) binds to a sequence (-40 to -54) located close to the TATA-box. Enterocytes from newborn pigs with high lactase activity contain high amounts of NF-LPH1, whereas enterocytes from adult pigs with low lactase activity contain low amounts of NF-LPH1. The liver does not contain lactase activity, and NF-LPH1 is not present in liver nuclear extracts in detectable amounts. This indicates that NF-LPH1 is involved in the decline of lactase at weaning and may be of importance for the molecular explanation of hypolactasia in humans. It was demonstrated by transfection of two different promoter-reporter gene constructs into Caco2 cells, that there are additional cis-element(s) in the region -142 to approximately -980, which are important for the transcription of the lactase-phlorizin hydrolase gene.