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In older lenses and particularly in the lens nucleus, a metazyme of the phosphofructokinase (PFK) is found, which had been formed by postsynthetic changes in conformation. Its kinetic and physicochemical properties are changed, and its activity is unstable under heat influence. A 5 krad X-irradiation of bovine lenses effects inactivation of the PFK. metazyme form comparable to that found in heat lability tests after raised temperature. An in vitro incubation also effects the protein conformation of PFK. Results indicate that the age-dependent protein conformation can be further modified by exogenous influences. The potentiation of postsynthetic changes in conformation may probably be considered as cooperation of additional risk factors, which might be subliminal in themselves, but accumulate in the sense of a syn- or cocataractogenesis. A coincidence of single changes would be the consequence.