A series of lens pairs (each pair from the same donor) from human eyebank eyes were incubated in a medium containing tracer leucine. One lens of each pair was exposed to long-wave ultraviolet light. The other was shielded. The irradiated lenses accumulated free leucine to a level only 53% of that in the shielded lenses. Irradiation depressed leucine incorporation to 29% of the control level. These results show that UV may adversely affect important parameters of human lens protein metabolism in vitro over a period of 10–30 h.