The pancreatic stone protein (lithostathine) secreted by the exocrine pancreas is an inhibitor of CaCO3 crystal growth. This protein, which is also present in endocrine pancreas, has also been called the regeneration protein (reg). Here we report the mapping of the REG gene to chromosome 2 using the polymerase chain reaction for the specific amplification of human reg sequences in rodent/human somatic cell hybrid DNA. A regional assignment has been made by in situ hybridization to metaphase chromosomes using two different fluorescently labelled genomic probes corresponding to the REG gene and a related gene REGL. Both probes hybridized to chromosome 2p12 suggesting the tandem organization of these genes.