A Pharmacokinetic (PK) Study of Natalizumab (Tysabri) at Steady State

Key Inclusion Criteria: * Ability to understand the purpose and risks of the study and provide signed and dated informed consent and authorization to use protected health information (PHI) in accordance with national and local subject privacy regulations. * Must be a multiple sclerosis (MS) patient enrolled in the Tysabri Outreach: United Commitment to Health Prescribing Program (TOUCH) who is not expected to discontinue Tysabri therapy prior to completion of the requirements of this study. * Must have been treated with monthly IV infusions of Tysabri 300 mg for at least 12 months, with the 9 most recent doses having been administered at 28±7 day intervals. * Must test negative for antibodies to Tysabri at the Screening Visit. * Must have a magnetic resonance imaging (MRI) brain scan, performed prior to the initiation of treatment with Tysabri, on file. * Must weigh between 42 and 126 kg, inclusive. * All male subjects and female subjects of childbearing potential must practice effective contraception during the study. Key Exclusion Criteria: * History of, or abnormal laboratory results indicative of, any clinically significant cardiac, endocrinologic, hematologic, hepatic, immunologic, metabolic, urologic, pulmonary, gastrointestinal, dermatologic, psychiatric, and renal, or other major disease, as determined by the Investigator. * Positive result for antibodies to Tysabri at any prior evaluation. * Treatment with an investigational product or approved therapy for investigational use within 6 months prior to the start of PK sample collection or during the course of this study. Concurrent participation in an observational study (e.g., Tysabri Global Observational Program in Safety \[TYGRIS\]) is permitted. * Pre-scheduled for any elective procedure or medical treatment during the study period. * History of drug or alcohol abuse (as defined by the Investigator) within 2 years prior to the Screening Visit. * Female subjects who are breastfeeding, pregnant, or planning to become pregnant while on study. * Alcohol use within 24 hours prior to the Baseline Visit. * Inability or unwillingness to comply with study requirements, including the presence of any condition (e.g., physical, mental, social) that is likely to affect the subject's ability to comply with the study protocol. * Other unspecified reasons that, in the opinion of the Investigator or Biogen Idec, make the subject unsuitable for enrollment. NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.