We have mapped the human gene for ferrochelatase (FECH; ferroheme-protolyase, EC 4.99.1.1) to chromosome 18 by hybridization of cDNA to sorted chromosomes. The probe was obtained by PCR-directed amplification of a human marrow cDNA library in lambda gt 10. Subchromosomal localization of ferrochelatase to 18q22 was determined by chromosomal hybridization in situ using a human ferrochelatase genomic clone in lambda EMBL 3 that contained a 20-kb insert. Since ferrochelatase activity is deficient in patients with the inherited disease erythropoietic protoporphyria, a locus for this disease may be assigned to 18q22, one of few monogenic defects that have been mapped to this chromosome.