The stability of bovine lens α-crystallin with respect to temperature, pH and urea has been investigated by 1H and 31P-NMR spectroscopy. The 1H and 31P-NMR spectra of α-crystallin show little change with temperature up to 75°C, indicating that α-crystallin has great thermal stability and does not undergo any major change in structure with temperature. 1H spectral studies of α-crystallin and its isolated αA and αB subunits reveal a marked difference in the stability of these species. It is found that, at pH 2.5, αA-crystallin adopts a native conformation whereas αB-crystallin is denatured. On the other hand, the two subunits when part of the total α-crystallin aggregate adopt a native conformation at pH 2.5, but in the presence of 0.1 M glycine the αB subunits become denatured. Thus, αA-crystallin and total α-crystallin are more stable species than αB-crystallin and, in total α-crystallin, there is an interaction between the compact domains of the αA and αB subunits that leads to enhanced stability. Finally, changes in the 1H and 31P-NMR spectra of αA-crystallin and αB-crystallin in the presence of varying concentrations of urea are consistent with a two-domain model for α-crystallin subunits with the C-terminal domain being less stable and unfolding first in the presence of urea. © 1993.