Increased acetaldehyde levels have been found in non-alcoholic liver diseases and an acetaldehyde-collagen adduct has been reported in rats with CCl4-induced cirrhosis. In cytosol and microsomes of rats with cirrhosis produced by N-nitrosodimethylamine, a similar acetaldehyde-protein adduct of approximately 200 kD was recognized by rabbit IgG raised against either an in vitro produced hemocyanin-acetaldehyde adduct or an in vivo occurring P4502E1-acetaldehyde adduct isolated from alcohol-fed rats, as well as by anti-rat collagen (I) IgG. Its immune complexes contained 3 proteins that reacted with the anti-collagen IgG and were digested by collagenase: 2 proteins with molecular weights similar to procollagens alpha 1 and alpha 2, and a beta 1,2(I)-like protein which was readily produced by in vitro modification of cytosol with acetaldehyde.