Nitrite treated nonheme protein was separated from the free nitrite of the reaction mixture by Sephadex G-15 column chromatography. Spectral analysis (absorbance at 330 nm) indicated that tryptophyl residues of the protein had been modified with the NO group. The protein-nitrite complex was not stable, decomposing faster at lower pH and higher temperature. As the extent of the decomposition increased with time, an increasing amount of nitrite was detected by the AOAC method, indicating that the Griess reagent reacted with nitrite after it was regenerated from the NO group. When myoglobin was incubated with the nitrosated protein, nitrosyl hemochrome could be extracted from the reaction mixture in the presence of ascorbic acid. The results indicate the potential reversibility of the protein-nitrite reaction.