Inherited variations in monoamine oxidase (MAO) activity are thought to affect human behavior and expression of disease. The present study has established the chromosomal location of one of the structural genes coding for this enzyme. Mapping was carried out by somatic cell hybridization between normal human skin fibroblasts and mouse neuroblastoma cells. Selective media for growth of cells with or without hypoxanthine phosphoribosyltransferase (HPRT) activity were used to obtain hybrid lines which had retained or lost the human X chromosome, respectively. Cytogenetic techniques, isozyme analysis, and limited proteolysis and peptide mapping of [3H]pargyline-labeled MAO were used to characterize hybrid lines. With one exception, only lines containing the human X chromosome and human forms of two X-linked enzymes (phosphoglycerate kinase and glucose-6-phosphate dehydrogenase) expressed the human form of the flavin polypeptide of type A MAO. The exceptional hybrid line contained a putative translocation of part of the human X chromosome, since it expressed human forms of both MAO and phosphoglycerate kinase but neither the human form of glucose-6- phosphate dehydrogenase nor HPRT activity. This evidence indicates that the structural gene for the flavin polypeptide of MAO-A is on the human X chromosome. This represents the first chromosomal assignment of a human gene coding for an enzyme of neurotransmitter metabolism. This information will help to elucidate the structure of MAO and modes of its inheritance in the human population.