Microsomal epoxide hydrolase (E.C.3.3.2.3) activity has been measured in human x mouse hybrid cells prepared from human cells expressing 6-7 x the activity of the mouse cells. Rabbit antihuman and antimouse antisera raised against purified enzymes were used to discriminate between human and mouse enzymes. All twenty five clones examined did not express human enzyme and this correlated with the loss of human chromosome 6 from each cell line. Four hybrids expressed 2-3 x the activity expressed by the mouse cell parent and these all retained more human chromosomes, specifically chromosome 19, than those with low activity. It is concluded that the human gene for epoxide hydrolase may be on chromosome 6 and that other gene products can affect the level of activity expressed by a cell.