The European Federation of Neurological Societies (EFNS) guidelines on the use of anti‐interferon beta neutralising antibody measurements put the spotlight on the importance of neutralising antibodies (NAbs) in multiple sclerosis (MS) therapy, and the need for effective testing and monitoring. Freelance medical writer Nicola O'Connell looks at the impact of NAbs on the efficacy of MS treatment and at the options for measuring NAbs. Copyright © 2007 Wiley Interface Ltd