A human neutralizing monoclonal antibody is directed against the VP1 protein of JC virus. The JC virus is responsible for progressive multifocal leukoencephalopathy (PML). The antibody is used in a therapeutic or prophylactic treatment of a JCV infection or of a disease associated with a JCV infection, such as progressive multifocal leukoencephalopathy (PML). The antibody is also used in the diagnosis of JCV infections or of diseases associated with JCV infections.