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Clone 46 was previously shown to represent mouse cytochrome P1-450 cDNA by both translation arrest experiments and segregation of induced P1-450 mRNA with induced aryl hydrocarbon hydroxylase activity among individual 3-methylcholanthrene-treated offspring of the (C57BL/6N)(DBA/2N)F1 X DBA/2N backcross. With clone 46 as a probe, a MOPC 41 mouse genomic-DNA library was screened. lambda 3NT12, a 16 X 10(3)-base-pair insert of genomic DNA grown in a recombinant Charon 4A lambda vector phage, was isolated and characterized. It was determined that clone 46 hybridizes to the extreme 5' end of lambda 3NT12. pMJE12, a 3.0 X 10(3)-base-pair fragment in the 5' region of lambda 3NT12, was subcloned in plasmid pBR322 and used as a probe to screen again the same mouse-DNA library; recombinant phages lambda 3NT13, lambda 3NT14, and lambda AhP-1 were isolated and characterized. The relative orientation of each of the four genomic clones on the mouse chromosome was determined. Only lambda AhP-1 contains the entire P1-450 genomic gene, which by R-loop analysis spans about 46 X 10(2) base pairs and contains at least five exons. Clone 46 is shown to be a 3' unique sequence of the genomic P1-450 gene. The lambda AhP-1 genomic-DNA clone from the MOPC 41 plasmocytoma is shown by a series of restriction enzymes to be the same as genomic DNA from normal mouse liver. With a subclone in the 5' portion of the P1-450 gene, two and three hybridizable fragments are found with mouse genomic DNA that has been digested with EcoRI and BamHI, respectively.