The primary objective of this study is to compare the pharmacokinetic (PK) and pharmacodynamics (PD) of single subcutaneous (SC) and intramuscular (IM) doses of 300 mg natalizumab to intravenous (IV) administration of 300 mg natalizumab in multiple sclerosis (MS) participants. The secondary objectives are to investigate the safety, tolerability and PK of repeated natalizumab doses administered SC and IM, to investigate the immunogenicity of repeated natalizumab doses administered SC and IM, to explore proof of concept within the secondary progressive multiple sclerosis (SPMS) population using change from baseline in clinical measures including: expanded disability status scale (EDSS), multiple sclerosis functional composite scale (MSFC), symbol digit modalities test (SDMT), visual analogue scale (VAS), and visual function test; and brain magnetic resonance imaging (MRI) measures including: number of new or newly-enlarging T2 hyperintense lesions, number of new T1 hypointense lesions, number of new gadolinium-enhancing (Gd+) lesions, whole brain atrophy, magnetization transfer ratio (MTR), and diffusion tensor imaging (DTI) and to observe the effect of natalizumab administered IV and SC on brain MRI measures in participants with relapsing forms of MS.