The hybridizabilities of the gamma-irradiated chicken heart and pig muscle lactic dehydrogenases were estimated by hybridizing the irradiated enzymes with the unirradiated pig heart lactic dehydrogenase. The disc gel electrophoretic patterns of the inter- and intraspecific hybrids showed that the LDH activity of the pig heart isozyme band increased as a function of dose. This observation was analyzed upon the binomial redistribution pattern of the recombined subunits. The result shows that the hybridizabilities of both the chicken heart and pig muscle isozymes decreased along with the loss of catalytic activity and the release from substrate inhibition. The titration of free SH groups of the irradiated chicken isozyme suggested that the unfolding of the peptide chain destroyed the specific tertiary structure needed for the binding of subunits.