Comparative analysis of cytosolic fraction distribution and conformational stability in lactate dehydrogenase (LDH) isozymes in healthy men of different age revealed significant differences indicating age-related characteristics of the enzyme. Activity of cytosolic fraction of LDH-1 and conformational stability of LDH-2 and LDH-3 in 40-year-old men were higher than in 20-year-olds. These data suggest age-related determination of conformational stability of individual LDH isozymes and functional heterogeneity of the enzyme within each isoform.