During the last twenty years, numerous biochemical aberrations in red blood cells of subjects suffering from various mental disturbances have been detected. Red blood cell abnormalities observed so far include changes in the activity of some membrane-bound enzymes and receptors, different levels of oxidative stress, and differences in the lipid composition and structure of the cell membrane. Some of these aberrations were observed in first-episode mentally ill patients, and correlate well with the severity of symptoms. They suggest the existence, even at the onset of illness, of general cell membrane alterations, which are believed to play a crucial role in the receptor-mediated transduction of neurotransmitters. We review biochemical and physical changes of the erythrocyte membrane occurring in the main mental disorders--schizophrenia, depression, and bipolar disorder--and discuss their pertinence to the "membrane theory of schizophrenia." We also discuss the possibility of using erythrocyte aberrations as potential additional tools in the diagnosis of psychiatric diseases.