The creatine kinase B (CK-B) subunit was determined by our own highly sensitive solid-phase direct immunoassay in three neurological diseases which represent models of central nervous system (CNS) involvement: (1) idiopathic epilepsy (18 patients), a disease characterised by a transient neuronal hyperactivity; (2) Parkinson’s disease (17 patients), a degenerative disorder of the CNS, and (3) multiple sclerosis (21 patients), which represents a model of demyelination of the CNS. A group of 50 controls was also studied. The results showed that in patients with epilepsy and multiple sclerosis the CK-B activity was considerably lower than in the controls (p < 0.01 and p < 0.005, respectively), while no differences were found between the controls and the patients with Parkinson’s disease. It is suggested that the low CK-B activity observed might be due to the medication taken or to the disease process itself. The present findings have to be considered as an indication for further CK isoenzyme studies in systematic disorders of the nervous sys