The cellular transcription factor SP 1 binds to critical regulatory elements in a variety of cellular and viral promoters. The gene encoding the ∼100-kDa SP 1 protein contains zinc finger DNA-binding domains and glutamine-activation domains. Since SP1 is involved in the regulation of a variety of cellular genes, we wished to determine its chromosomal localization. Southern blot analysis of genomic DNA from a panel of mouse × human somatic cell hybrids indicated that SP1 was localized to chromosome 12. In situ hybridization allowed the localization of the gene encoding SP 1 to human chromosome 12q12→q13.2, with 12q13.1 being the most probable location.