The Ehrlich reaction was optimized to determine pyrrolized proteins produced as a consequence of lipid peroxidation and oxidative stress. The procedure consisted of the treatment of the modified protein with p-(dimethylamino)benzaldehyde at a controlled acidity and temperature, and the determination of adducts produced against the blank obtained in the absence of the reagent. The extinction coefficient of Ehrlich adducts was calculated by using epsilon-N-pyrrolylnorleucine (Pnl) as standard and was 35,000 M-1 cm-1. The response was linear and reproducible within the range 0. 16-20 microM Pnl. The assay was applied to determination of pyrrole content in bovine serum albumin, bovine alpha-globulins, bovine gamma-globulins, and mixtures of them, incubated overnight with 1 mM of 4,5(E)-epoxy-2(E)-heptenal, obtaining results similar to those from determination of Pnl by capillary electrophoresis after basic hydrolysis of the protein. The method was also applied to pyrrole determination in bovine plasma proteins either incubated with epoxyalkenals, hydroxyalkenals, lipid hydroperoxides, and secondary products of lipid peroxidation, or oxidized with Fe3+/ascorbate. All these treatments produced pyrrolization of plasma proteins and all Ehrlich adducts gave very similar absorbance spectra with the exception of that produced in the treatment with hydroxyalkenals. The above results suggest that protein pyrrolization is a normal consequence of the lipid peroxidation process and of oxidative stress, and that Ehrlich adducts may be valid to determine this pyrrolization.