The primary objective of the study is to determine the pharmacokinetic (PK) profile of multiple doses of natalizumab in pediatric subjects with relapsing-remitting multiple sclerosis (RRMS). The secondary objectives are as follows: to characterize the pharmacodynamic (PD) profile of natalizumab (as defined by α4 integrin binding) and to explore the safety and tolerability of multiple doses of natalizumab in the pediatric population.