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This thesis describes investigations on the biochemical and genetical properties of human intestinal lactase-phlorizin hydrolase which were designed to help in the understanding of the molecular basis of the lactase persistence/non-persistence polymorphism in man. There were two major objectives, one was to study the expression of the lactase gene in lactase persistent and non-persistent individuals and the second was to identify as many polymorphisms as possible within the lactase gene in order to increase informativeness for linkage analysis and to explore the extent of allelic association across the gene. The expression of the lactase gene was studied in a series of samples of adult intestine. The level of the enzyme activity, the protein and the mRNA was examined in each sample. Individuals were assigned lactase persistence status on the basis of their lactase to sucrase activity ratio. The results of this analysis suggest that differences in the level of transcription of the lactase gene are important in determining the lactase persistence phenotype. Lymphoblasoid cell lines were established from 32 of these individuals and used to prepare genomic DNA. The mapping of the lactase gene to chromosome 2 was confirmed and refined to band q21 using a panel of somatic cell hybrids and in situ hybridisation. Using a variety of electrophoretic techniques which are sensitive to the detection of single base changes, polymorphisms were identified at seven different sites within the 70kb region comprising the lactase gene. Analysis of these polymorphisms in 50 families revealed that only 3 of the possible 128 haplotypes occur frequently, suggesting an area of linkage disequilibrium stretching across the whole lactase coding region. These markers have also been used for linkage analysis and enabled the identification of the most closely linked hypervariable marker (D2S44). The lactase gene polymorphisms are currently being used to study unrelated individuals as well as families characterised with respect to their lactase persistence/non-persistence status and preliminary results of this analysis are described.