Coproporphyrinogen oxidase (EC 1.3.3.3) catalyzes the sixth step in the heme biosynthetic pathway, the oxidation of coproporphyrinogen III to protoporphyrinogen IX. The activity of this enzyme is deficient in the disease hereditary coproporphyria. The sequence of the cDNA and predicted amino acid sequence of the human coproporphyrinogen oxidase are presented. The human protein sequence contains a region completely homologous to that we obtained by sequencing an 11-amino acid peptide fragment from purified murine liver coproporphyrinogen oxidase. Results of Southern blotting were consistent with the presence of a single human coproporphyrinogen oxidase gene, and Northern blotting demonstrated one transcript of similar size in erythroid and nonerythroid cell lines. Expression of the cDNA coding for the putative mature human coproporphyrinogen oxidase in Escherichia coli resulted in a 17-fold increase in coproporphyrinogen activity over endogenous activity.