A cDNA from a chick liver library that encodes for haem oxygenase has been cloned and sequenced. Positive clones were identified with monospecific antibodies to the purified enzyme from chick liver and a cDNA of rat haem oxygenase-1. The length of the cDNA is 1258 bases. An open reading frame of 888 bases was identified by comparison of nucleotide and amino acid sequences with those previously identified for haem oxygenase of mammalian or avian origin. The protein corresponding to this fragment of DNA is composed of 296 amino acid residues and has a molecular mass of 33,509 Da, which is similar to that previously estimated for haem oxygenase purified from chick liver. Unequivocal identification of this clone as that complementary to haem oxygenase was provided by (a) comparison of amino acid compositions and partial sequences with those previously established for the purified enzyme, (b) comparison with nucleotide and amino acid sequences for haem oxygenase from rat and human sources and (c) expression in Escherichia coli with production of high levels of mRNA, protein and haem oxygenase activity after exposure of the transfected bacteria to isopropyl beta-D-thiogalactopyranoside (IPTG). Overall, the similarity of chick haem oxygenase to rat and human haem oxygenase (nucleotides 66% and amino acids 62%) is moderately high. The region between proline-129 and alanine-157 is identical in all three enzymes, including histidine-135, which is proposed to play a key role in binding the substrate haem at the active centre of the enzyme. Northern blots also show that treatment of chicks with CdCl2, a potent inducer of haem oxygenase, results in increases in 1.65-1.70 kb mRNA, which hybridizes selectively to the full-length cDNA or to a synthetic 24-base oligonucleotide with sequence identical to that of a portion of the haem oxygenase cDNA. These results suggest that Cd-dependent induction of haem oxygenase is due to increased transcription of the gene or stabilization of its message.