SUMMARYMice undergoing transplantation (runt) disease produced by injections of spleen cells across the H-2 locus in various strain combinations show a dramatic elevation of renal lysozyme activity. This enzyme would seem to be a good biochemical marker of the graft-versushost reaction and related phenomena under controlled experimental conditions. An increased renal lysozyme activity also occurs in “normal” mice with the onset of advanced age. This age-related increase in activity of the enzyme can be interpreted as supportive evidence for the immunologic theory of aging.