The human c-fos proto-oncogene was mapped by in situ hybridization to chromosome 14, band q24.3-q31. As a probe we used 3H-labelled mouse c-fos RNA transcribed in vitro by SP6-RNA-polymerase. Of a total of 40 grains on chromosome 14, 19 (47.5%) were located within the 14q24.3-q31 region (p much less than 0.001). Mechanisms for c-fos to gain transforming ability by a break in a critical part of the gene have recently been described. Thus there are several possibilities for chromosomal aberrations within the 14q24.3-q31 region to be responsible for c-fos deregulation and which may result in neoplastic growth. Such specific aberrations are found in a variety of human neoplasms.