In the present study, we characterize the covalent modification of a protein by crotonaldehyde, a representative carcinogenic aldehyde, and describe the endogenous production of this aldehyde in vivo. The crotonaldehyde preferentially reacted with the lysine and histidine residues of bovine serum albumin and generated a protein-linked carbonyl derivative. Upon incubation with the histidine and lysine derivatives, crotonaldehyde predominantly generated beta-substituted butanal adducts of histidine and lysine and N(epsilon)-(2,5-dimethyl-3-formyl-3,4-dehydropiperidino)lysine (dimethyl-FDP-lysine) as the putative carbonyl derivatives generated in the crotonaldehyde-modified protein. To verify the endogenous formation of crotonaldehyde in vivo, we raised the monoclonal antibody (mAb82D3) against the crotonaldehyde-modified protein and found that it cross-reacted with the protein-bound 2-alkenals, such as crotonaldehyde, 2-pentenal, and 2-hexenal. The anti-2-alkenal antibody recognized multiple crotonaldehyde-lysine adducts, including dimethyl-FDP-lysine and an unknown product, which showed the greatest immunoreactivity with the antibody. On the basis of the chemical and spectroscopic evidence, the major antigenic product was determined to be a novel Schiff base-derived crotonaldehyde-lysine adduct, N(epsilon)-(5-ethyl-2-methylpyridinium)lysine (EMP-lysine). It was found that the lysine residues that had disappeared in the protein treated with crotonaldehyde were partially recovered by EMP-lysine. The presence of immunoreactive materials with mAb82D3 in vivo was demonstrated in the kidney of rats exposed to the renal carcinogen, ferric nitrilotriacetate. In addition, the observations that the metal-catalyzed oxidation of polyunsaturated fatty acids in the presence of proteins resulted in an increase in the antigenicity of the protein indicated that lipid peroxidation represents a potential pathway for the formation of crotonaldehyde/2-alkenals in vivo. These data suggest that the formation of carcinogenic aldehydes during lipid peroxidation may be causally involved in the pathophysiological effects associated with oxidative stress.