Genetic variation in the number and reactivity of beta globin sulfhydryl groups causes variation in erythrocyte redox status in mouse populations. These experiments demonstrate the use of capillary isoelectric focusing for measuring endogenous S-glutathionyl hemoglobin and identifying mouse beta globin (Hbb) haplotype in inbred and outbred mouse strains with mono-cysteinyl or di-cysteinyl beta globins. Hbb haplotype can be readily determined in all strains based on characteristic differences in peak profiles or on peak mobility shift induced by thiol exchange with glutathione disulfide in vitro. This method could prove useful for in vivo study of factors that influence thiol protein modification.