Human alpha-mannosidase activity (alpha-D-mannoside mannohydrolase, EC 3.2.1.24) from tissues and cultured skin fibroblasts was separated by gel electrophoresis into a neutral, cytoplasmic form (alpha-mannosidase A) and two closely related acidic, lysosomal components (alpha-mannosidase B). Human mannosidosis, an inherited glycoprotein storage disorder, has been associated with severe deficiency of both lysosomal alpha-mannosidase B molecular forms. Chromosome assignment of the gene coding for human alpha-mannosidase B (MANB) has been determined in human-mouse and human-Chinese hamster somatic cell hybrids. The human alpha-mannosidase B phenotype showed concordant segregation with the human enzyme glucosephosphate isomerase (GPI) (D-glucose-6-phosphate ketolisomerase, EC 5.3.1.9) but discordant segregation with 30 other enzyme markers representing 20 linkage groups. The glucose-phosphate isomerase gene has been assigned to chromosome 19 in man. This MANB-GPI linkage and confirming chromosome studies demonstrate assignment of the alpha-mannosidase B structural gene to chromosome 19 in man. Since mannosidosis is believed to result from a structural defect in alpha-mannosidase B, these findings suggest that the mannosidosis mutation is located on chromosome 19 in man.