Isocitrate lyase from both young and old free living nematodes (Turbatrix aceti) has been purified and compared. The "old" enzyme consists of the same five isozymes as the "young" preparation, but with quantitative differences. The enzyme shows an age-related decline in specific activity. Use of antibodies has confirmed the accumulation of cross-reacting material in old organisms as found by Gershon and Gershon ((1970) Nature 227, 1214), using crude homogenates. Km, molecular weight, subunit size, and behavior toward the inhibitors oxalate, malonate, and tartronate all appear unchanged. Although the enzyme isolated from old organisms has a sharply reduced specific activity, it binds as well to an affinity column as does "young" enzyme. It is tentatively concluded that the loss of specific activity in the "old" enzyme is due to the presence of partially active molecules, rather than to a mixture of active and inactive molecules.