Sex-related differences in the expression of glutathione S-transferase (GST) isozymes of mouse liver have been described. There were no apparent qualitative differences in the isoelectric focusing profiles of the GST isozymes from male and female mouse liver. Both male and female mice have at least four GST isozymes in their liver with pI values of 9.8, 8.7, 6.4 and 5.7. Kinetic, immunological, and structural properties including the N-terminal region amino acid sequences of these isozymes have been determined and they have been classified into alpha, mu, and pi classes. The most cationic isozyme (pI 9.8) belongs to the alpha class and is comparatively more abundant in female liver. The isozyme having pI 8.7 belongs to the pi class and is more abundant in male liver. The mu class GST pI 6.4 as well as the isozyme having pI 5.7 which corresponded to the a class isozyme GST 8-8 of rat liver were more abundant (about 1.5-fold) in male mouse liver as compared to the female. Interestingly, present studies reveal sex-related differences in the heat stabilities of the alpha and pi class GSTs of mouse liver. The alpha class GST pI (9.8) isolated from female mouse liver was more thermostable as compared to the corresponding enzyme from male mouse liver. On the contrary, the pi class GST (pI 8.7) from male mouse was more thermostable as compared to the corresponding enzyme from the female mouse.