The irreversible denaturation of hen egg white lysozyme was monitored by one-dimensional H-1 nuclear magnetic resonance (1D H-1 NMR). Extended incubation of lysozyme at 75 and 81 degrees C in 20% why PEG 1000 caused more native lysozyme peak intensity loss than at 0% PEG. Further an increase in lysozyme concentration from 3.5 to 10 mM increased the irreversible denaturation as did the increase in temperature from 75 to 81 degrees C. Additionally, new peaks appeared in most spectra. Intriguingly, spectra in PEG after extended heating all resembled the reversible denatured state, whereas at 0% PEG spectra appeared native at reduced intensity, indicating that PEG causes the denatured state to persist, while without PEG, aggregation may persist. The methylenes and ethers of PEG may paralyze the denaturation and delay aggregation by hydrophobic and hydrogen bond interactions with the denatured state. The results indicate that NMR techniques may have many potential uses in biopharmaceutical industries. NMR may be able to provide further insight into product integrity, such as the extent of irreversible denaturation.