The specific activity of protein L-isoaspartyl methyltransferase, an enzyme implicated in the metabolism of damaged, isoaspartate-containing proteins, has been measured in postmortem samples of parietal cortex from 30 individuals (19 with Alzheimer's disease and 11 controls). Methyltransferase specific activity was positively correlated with age at death, increasing by 2.9 pmol/min/mg of protein for every ten years of age (r = .51, p less than 0.005). This correlation was significant in the control and Alzheimer's disease groups alike. Specific activity also appeared to be about 15% higher in females than in age- and diagnosis-matched males (p less than 0.05). No significant differences were observed between age- and sex-matched Alzheimer patients and controls, suggesting that a deficiency in this enzyme is not responsible for the accumulation of abnormal proteins in Alzheimer's disease.