The content of casein kinase 2 is considerably decreased in ribosome-free extracts of the frontal cortex of schizophrenic and Alzheimer's disease patients in comparison to normal brains as has been demonstrated by means of immunoblotting. The activity of casein kinase 2 towards endogenous substrates and casein is also diminished in the cases of mental pathologies examined. This phenomenon may explain the well-known aberrations in the phosphorylation of structural proteins of human brain which are intrinsic for the mental diseases.