STUDY POPULATION: Study population will consist of 49 patients with relapsing remitting or secondary progressive MS with superimposed relapses who have been treated with Interferon beta at fully tolerated dosages for at least six months, and 49 healthy volunteers matched for gender, age and educational level. We expect to screen 65 individuals per arm in order to be able to enroll 49 individuals per arm, thus meeting the requirements of the sample size calculations.