The human cell surface antigen, CD24, is a glycosyl phosphatidylinositol (GPI)-linked glycoprotein that has been implicated in the differentiation and activation of granulocytes and B lymphocytes. Changes in expression of the antigen occur at critical times during B lineage development. CD24 was cloned by its homology to mouse heat-stable antigen. Southern blot analysis suggested the presence of multiple CD24 related sequences in the human genome. We have now mapped CD24 homologous sequences to chromosomes 6q21, 15q21-q22, and Yq11 by screening a panel of somatic cell hybrid DNAs and by in situ hybridization. At least two additional homologues, one located on chromosome 1 at band p36 and one tentatively mapped to chromosome 20, that are distantly related to CD24 were identified. Southern analysis of male and female DNA samples confirmed the presence of CD24 homologous sequences on the human Y chromosome. Sequencing of DNA fragments amplified from monochromosomal somatic cell hybrids showed that the CD24 cDNA was derived from a transcript originating from a gene on chromosome 6. The CD24 gene on the Y chromosome had many base changes compared to the cDNA but had retained an open reading frame, leaving open the question of whether this gene is functional. Both ATGs in the translation initiation region of CD24 homologous sequences on chromosome 15 were converted to GTGs, making it unlikely that this gene, if functional, encodes a protein similar to CD24. CD24, therefore, is a member of a multigene family, but it remains to be determined whether any of the related genes are functional.