The gene for human prothrombin, or factor II (F2) has been assigned to 11p11-q12 by the combined use of a panel of somatic cell hybrid DNAs and in situ hybridization, using both cDNA and genomic probes. In addition, the cDNA probe for F2 recognizes a homologous sequence which has been tentatively mapped to the X chromosome. Similar approaches have been used to confirm the assignment of the ceruloplasmin gene, but to regionally localize it more proximally than previously reported (3q21-q24). These results provide further evidence that genes encoding the coagulation factors and related proteins are dispersed throughout the human genome.