Fatty acid ethyl esters are a family of non-oxidative metabolites of ethanol present in many tissues after ethanol consumption. In this report we demonstrate the existence in human liver of an acyl-CoA:ethanol acyltransferase activity which may be responsible in part for the synthesis of these compounds in vivo. The effects of oleoyl-CoA and ethanol concentrations, presence or absence of bovine serum albumin and detergent, pH and enzyme concentration on this activity have been determined. Acyl-CoA:ethanol acyltransferase activity is localised in the membrane-bound fraction. Using inhibitors directed against related enzyme activities, it has been shown that the activity is not related to serine-dependent carboxylesterases or acyl-CoA:cholesterol acyltransferase, but tht it may be associated with acyl-CoA hydrolase activity. We have also compared acyl-CoA:ethanol acyltransferase activity with fatty acid ethyl ester synthase activity in microsomes and cytosol from the same liver. Our data indicate that these activities are comparable in vitro (on a unit/g liver basis), and suggest that both may be significant in vivo.