The erythrocytes from various types of psychiatric patients, their apparently normal family members and normal controls were incubated in synthetic serum containing lithium. The lithium concentration in erythrocytes-to-serum (Li index) was measured as an index of lithium transport. The following results were obtained. 1) No sex difference was present in the Li index within the psychiatric group, their family members or normal controls. Psychiatric patients showed a significant positive correlation between Li index and age, but such a relationship was not evident in either the family members or normal controls. 2) Li indices showed wide variations within the psychiatric population and within the normal group but appeared relatively constant within members of the same family. Thus, a genetic factor was suspected as operating in lithium permeability across the erythrocyte membrane. 3) In schizophrenic patients, a positive tendency was noted between a high intrafamilial morbidity and a higher Li index. Schizophrenic patients with particularly high hereditary load showed a significantly higher Li index than the control values. Female patients of this group indicated marked Li indices high above the range for normal individuals. 4) Schizophrenic patients in hyperkinetic states showed lower Li indices than those in hypokinetic states. Hospitalized manic-depressive psychotic patients showed a tendency for low Li indices, but during remission, these patients showed higher Li indices. 5) In the combined psychiatric population, an 'apparent inverse relationship was found between extremely high serum creatine phosphokinase (CPK), glutamic oxalacetic transaminase (GOT) and glutamic pyruvic transaminase (GPT) activity and a low Li index. Patients with CPK activities above 100 units had significantly lower Li indices compared with those with CPK activities of less than 60 units. 6) CPK and Li index reflect different measures and can not be treated on the same basis, but it can be presumed that lithium transport outside erythrocytes in more active when serum CPK activity is abnormally high. Thus, the possibility is suggested here that permeability across the erythrocyte membrane varies according to psychiatric states. 7) In schizophrenic patients, no significant correlation was found between the Li index and dosages of antipsychotic drugs excluding lithium, the duration of medication or the duration of illness.