The chromosomal localization of the human Mu class glutathione S-transferase (GST) genes has been complicated by two factors; the total number of genes is unknown and there is a polymorphism that results from the presence or absence of the GSTM1 gene. Three human Mu class glutathione S-transferase isoenzymes, GSTM1, GSTM2, and GSTM3, have been characterized previously, and we have recently cloned and characterized GSTM4, another member of this class. Here we report that a probe derived from GSTM4 cross-hybridizes with the other three known human Mu class GST genes. In situ hybridization with the GSTM4 probe localized a major region of hybridization on chromosome band 1p13. Although there is a region of very weak hybridization on chromosome 6, these data indicate that the human Mu class gene family is largely clustered and not dispersed on different chromosomes. The identical hybridization patterns in individuals with or without the GSTM1 gene suggest that this locus is a component of the Mu class GST gene cluster.