A human cytochrome P-450 cDNA clone, lambda hPA6, was isolated from a human adult liver cDNA library. The expression plasmid pHM3 delta 2 was constructed by insertion of lambda hPA6 and a synthetic DNA into the yeast mRNA and reduced carbon monoxide-difference spectra of the yeast microsomes revealed that only the yeast transformed by the expression plasmid pHM3 delta 2 produced foreign cytochrome P-450. In addition, the expressed protein reacted with antiserum to P-450-HM2, a form probably identical with P-450MP, and showed a detectable level of mephenytoin 4-hydroxylase activity.