Transforming growth factors (TGF) are defined as biologically active polypeptides which reversibly confer the transformed phenotype onto untransformed cultured cells. They have been subdivided into two classes: type alpha and type beta TGFs. TGF-beta acts synergistically with TGF-alpha in inducing phenotypic transformation. TGF-beta can also act as a negative autocrine growth factor. A human 1050-bp EcoRi cDNA fragment was used to map the human locus for TGF-beta by Southern blotting of DNA prepared from 17 human X Chinese hamster somatic cell hybrids. The human-specific restriction fragments segregated with human chromosome 19 in all of 14 informative hybrids. All other human chromosomes were discordant with the TGF-beta bands in at least four hybrids. After in situ hybridization of the tritiated TGF-beta probe to normal human metaphase spreads, 151 silver grains were scored in 54 cells. Of 24 grains over chromosome 19, 16 grains (11%) lay over region 19q13.1----q13.3. Of the 54 cells analyzed, 16 (30%) had label over region 19q13.1----q13.3. Thus, TGFB is assigned to chromosome 19, subbands q13.1----q13.3. The Tgf-beta locus in the mouse was mapped to chromosome 7 by hybridizing a murine cDNA probe to a Chinese hamster X mouse hybrid panel. Human chromosome 19 and proximal mouse chromosome 7 share another four homologous loci.