29 pairs of same-donor human lenses were incubated 6 h in radioactive leucine. The accumulation (CL/CM) and incorporation (cpm/mg protein) were measured. Donor ages were 4–86 years old. The total storage time was recorded. Same-donor (mated) pairs gave highly concordant results for the two members of a pair but there was extreme variability between different pairs, even when the age was comparable. Storage time did not appear to be a significant factor during the first 24 h but beyond that time continued storage led to marked deterioration of lens metabolism. Because of high variability, no significant dependence of metabolism on age was apparent. A consideration of these results along with comparable investigations from the literature leads to the conclusion that differences among individuals are the chief cause of the wide variation among human lenses although there is some evidence that storage conditions may produce a significant effect. It was not possible to derive a normal value for the parameters measured in this investigation.