1. The effect of Al3+ on superoxide dismutase in vitro was studied, since in uraemia there is excessive superoxide production and frequently an elevated serum Al3+ level. Thus, the protective role of superoxide dismutase is particularly important. 2. Al3+ in concentrations similar to those found in the serum of uraemic patients inhibits superoxide dismutase activity. The degree of inhibition is directly proportional to the Al3+ level. 3. The combination of excessive oxygen free radical production with an increased Al3+ level may contribute to a variety of complications, including aluminium dementia or initiation and promotion of carcinogenic processes, which are known to be more common in uraemic patients.