Patients with Alzheimer's disease (AD) have been reported to have abnormalities in the levels and activities of protein kinase C (PKC) in brain and other tissues. We have measured Ca2+-activated, phospholipid-dependent PKC activities and levels in cerebral cortex from frontal, motor, temporal and parietal regions, as well as in leukocytes and platelets from AD patients and controls. No significant differences in PKC histone H1 phosphotransferase activity were seen in frontal, motor, temporal or parietal cortex, or in leukocytes and platelets from AD patients and controls. Elevated PKC protein was present in cytosolic fractions from frontal cortex, but not in other brain regions, or in leukocytes and platelets. These data suggest that abnormalities of PKC phosphorylating activity are absent in AD.