The role of free radicals in protein modification and the importance of anti-4-hydroxy-2-nonenal (HNE) antibodies as marker of HNE-mediated cell toxicity has been well documented. Proteins modified by HNE in vitro, prior to immobilization on ELISA plates, have served as substrates for assaying these antibodies. We found preferential binding of HNE-modified versus unmodified proteins to ELISA plates and this prompted us to seek a more reliable assay. We report a method to HNE-modify any cysteine/histidine/lysine-containing protein or multiple antigenic peptide (MAP) following their immobilization on an ELISA plate. To a set of wells, HNE (200 microM) dissolved in PBS is added and incubated for 4 h, followed by regular ELISA. Since HNE was supplied dissolved in ethanol, PBS with appropriate amount of ethanol added was used as control. For inhibition experiments, HNE is incubated with or without inhibitors and then added to the wells. The commercial anti-HNE serum bound only to HNE-modified antigens. Sera from rabbits and mice immunized with HNE-modified 60 kDa Ro autoantigen preferentially bound the modified antigens. Modification of solid phase antigens in this manner makes assaying anti-HNE antibodies unambiguous. Lengthy dialysis procedures or the use of spin columns that lead to antigen loss becomes unnecessary for the separation of free HNE. We were able to HNE-modify various antigens (BSA, the autoantigens Ro, La and Sm/nRNP, 60 kDa Ro and Sm MAPs) using this procedure. Using MAPs, we confirmed the importance of histidine, lysine and cysteine residues in HNE modification. In addition, this method allowed identification of inhibitors of HNE-modification. We obtained 61%, 70% and 74% inhibition of HNE-modification of solid phase Ro MAP 166 substrate using BSA, Ro MAP 482 and Ro MAP 166, respectively. Glycyl-proline dipeptide and a MAP from the Sm autoantigen (PPPGMRPP) showed 0% inhibition of HNE-modification.