Three monoclonal antibodies reactive with human myelin basic protein have been used to develop immunoradiometric assays for this protein. Clone 1, a mouse IgG2a, is reactive with an epitope in the region 129-138. Clone 2, a mouse IgG1, is reactive with the region 119-131. Clone 12, a rat IgG, is reactive with the region 86-96. Competition experiments show that the reactions of Clone 1 and Clone 2 are mutually exclusive, probably because of steric effects. In contrast, when either Clone 1 or Clone 2 react they cause minimal interference with the subsequent binding of Clone 12. Less than 1 ng/ml of myelin basic protein can be detected in each of the two immunoradiometric assays developed. Clone 12 can also be used on its own in a competitive immunoassay to detect around 2 ng/ml. Using an extraction technique before the assay, serum or plasma can also be investigated. Assays for defined regions of myelin basic protein should prove valuable in identifying the products of myelin catabolism in patients with demyelinating disease.