The effect of aging on cellular response to toxic levels of metal ions was studied. It was found that for several metals the relative toxicity to human cells was: Zn/Cu/Mn/Mg. Further studies with Zn and Mg revealed no significant difference in cellular toxicity with increasing passage levels (in vitro aging). The effect of aging on sister chromatid exchange induction by fluorescent light was examined. Sister chromatid exchange levels were decreased in late passage human cells. It was also found that a low pretreatment dose of fluorescent light reduced SCE induction by later doses. Studies on bone formation and aging have shown that the histological changes and the uptake of radio-Ca run parallel in young and old rats, being greatly retarded in old animals.