Neurofilaments are the main cytoskeletal constituents in neuronal cells. They are belived to be important for maintaining the structural integrity and calibre of axons and dendrites thereby influencing the conduction velocity of nerve impulses.The neurofilament chains are divided into three groups according to their molecular size, neurofilament light (NF-L), neurofilament medium (NF-M) and neurofilament heavy (NF-H). The neurofilaments are obligate heteropolymers in vivo in which NF-L forms the backbone to which the heavier chains copolymerize to form the 10 nm neurofilament fibre. Different degenerative processes in the brain raise significant interest owing to the increasing mean age in the western world. Such diseases include amyotrophic lateral sclerosis, vascular dementia, frontal lobe dementia, progressive supra-nuclear paralysis, multiple system atrophy, low pressure hydrocephalus, and multiple sclerosis (MS). We have been able to generate six highly specific monoclonal antibodies for NF-L, and four independent epitopes were elucidated using Biacore and V8 protease degradation. Antibody 2:1 and 47:3 were selected components in a two-site ELISA assay for detection of NF-L in body fluids owing to their outstanding abililty to bind the antigen. The assay has a least detectable dose of 60 ng/l and a standard range of 60 to 64 000 ng/l. The assay was validated on its ability to detect changes of NF-L levels in CSF in patients with different neurological diseases. These were cerebral infarction, amyotrophic lateral sclerosis, relapsing remitting MS, extrapyramidal symptoms, and late onset Alzheimer’s disease. All the patient groups displayed significantly elevated NF-L levels as compared to the controls. We also tested the assay’s ability to monitor the amount of axonal breakdown in an animal model of MS. The NF-L levels were found to be elevated in rodents with chronic experimental autoimmune encephalomyelitis, giving a possible tool for monitoring new treatment strategies for axonal protection in MS. When studying a large population based MS material, we found axonal breakdown to be present early in the disease course and the breakdown was observed both in active relapse and clinically stable disease, indicative of ongoing neurodegeneration. NF-L levels were correlated to progression index, that is, high NF-L levels detected early in disease predict a fast progression of the disease. The amount of glial fibrillary acidic protein, a cytoskeletal protein found in astrocytes, was also quantified and was shown to be a good marker for the more progressive MS subtypes, that is, primary progressive and secondary progressive disease, indicating formation of astrocytic scars and activation of astrocytes. The test dealt with in this thesis has the potential to identify the slow chronic degenerative diseases with progressive disappearance of nerve cells and their large myelinated axons. There is a significant need clinically to be able to quantify such types of cell degeneration in relation to the progressive disappearance of nerve functions and to relate these different conditions to treatment regimens, disease progress, and prognosis.