The presence or absence of the human gene for the fifth component of complement (C5) was analysed in 19 human-rodent hybrid cell lines by hybridization to a radiolabelled probe derived from a human C5 cDNA clone. The segregation of C5 in these hybrids suggested that the gene is localized on chromosome 9, in the region 9q21-9qter. In situ hybridization refined the assignment of C5 to chromosome 9q22-33.