The murine Ah locus regulates de novo induction of liver microsomal cytochrome P1-450 by polycyclic aromatic compounds such as 3-methylcholanthrene (MC). The 23S mRNA for P1-450 has been shown to be induced by MC in Ah-responsive C57BL/6N mice. Using partially purified 23S mRNA from MC-induced mice, a double-stranded complementary DNA was synthesized and inserted into the Pst I site in pBR322 plasmid DNA. One clone which was positively identified as containing part of the P1-450 gene had an 1100 bp insert. Labelling of isolated insert by nick-translation has allowed us to identify a closely related sequence in human placenta DNA. This human equivalent has now been cloned in the phage vector Charon 16A.