The baculovirus expression system is suitable for functional expression of gastric H+,K(+)-ATPase. Expression of functional H+,K(+)-ATPase in Sf9 cells is accompanied by synthesis of large amounts of non-functional subunits. When H+,K(+)-ATPase is synthesised in the presence of 150-250 mM ethanol in the culture medium, two to threefold higher levels of functional H+,K(+)-ATPase are produced due to the formation of more functional subunits rather than to an increase of subunits per se. The catalytical properties of the ethanol-produced H+,K(+)-ATPase are indistinguishable from control preparations. The mechanism by which ethanol stimulates the formation of functional H+,K(+)-ATPase probably involves a direct effect on the physical properties of Sf9 membranes. In addition there also might be an indirect effect through ethanol inducible stress proteins acting as molecular chaperones.