The gene LCT which codes for the intestinal disaccharidase lactase-phlorizin hydrolase has previously been mapped, using somatic cell hybrids and linkage analysis, using the CEPH families, to chromosome 2. We describe here the regional localization of LCT to chromosome 2q21 by polymerase chain reaction (PCR) analysis of somatic cell hybrids and in situ hybridization. LCT is closely linked to D2S44, with a lod score of 30.6 at theta = 0.10.