• Total number of Gd-enhancing MRI lesions. [ Time Frame: 4, 8, 12, 16, 20, 24, 30, 36, 42, and 48 weeks ] [ Designated as safety issue: No ]
• Total number of new or newly enlarging T2-weighted MRI lesions. [ Time Frame: 4, 8, 12, 16, 20, 24, 30, 36, 42, and 48 weeks ] [ Designated as safety issue: No ]
• Time to first relapse. [ Time Frame: Weeks 24, 48 and period in between. ] [ Designated as safety issue: No ]
• Proportion of relapse-free subjects. [ Time Frame: Weeks 24, 48 and period in between. ] [ Designated as safety issue: No ]
• Proportion of subjects with anti-LY2127399 antibodies. [ Time Frame: 24, 48, and 72 weeks ] [ Designated as safety issue: No ]
• Pharmacodynamics of selected peripheral B cell subsets. [ Time Frame: Week 72 ] [ Designated as safety issue: No ]
• Serum pharmacokinetics (AUC). [ Time Frame: 42 weeks ] [ Designated as safety issue: No ]
• Expanded Disability Status Scale (EDSS). [ Time Frame: 12, 24, and 48 weeks ] [ Designated as safety issue: No ]
• Multiple Sclerosis Functional Composite Scale (MSFC). [ Time Frame: 12, 24, and 48 weeks ] [ Designated as safety issue: No ]
• Visual Analog Scale (VAS) of Wellbeing. [ Time Frame: 12, 24, and 48 weeks ] [ Designated as safety issue: No ]
• Medical Outcomes Study 36-Item Short Form Health Survey (SF-36). [ Time Frame: 12, 24, and 48 weeks ] [ Designated as safety issue: No ]
• 16-Item Quick Inventory for Depressive Symptomatology Self Report (QIDS-SR16). [ Time Frame: 12, 24, and 48 weeks ] [ Designated as safety issue: No ]
• Total number of new Gd-enhancing MRI lesions. [ Time Frame: 4, 8, 12, 16, 20, 24, 30, 36, 42, and 48 weeks ] [ Designated as safety issue: No ]
• Total volume of T2-weighted MRI lesions. [ Time Frame: 4, 8, 12, 16, 20, 24, 30, 36, 42, and 48 weeks ] [ Designated as safety issue: No ]
• Annualized relapse rate. [ Time Frame: Weeks 24 and 48. ] [ Designated as safety issue: No ]

To look at the ability of LY2127399 to reduce magnetic resonance imaging (MRI) lesions at 12, 16, 20, and 24 weeks compared to placebo.

• Drug: LY2127399
• Drug: Placebo

Inclusion Criteria:

• 18 through 64 years of age diagnosed with relapsing-remitting multiple sclerosis (RRMS), who can walk without aid or rest for at least 200 meters (approximately 1/10 of a mile).
• Women who can become pregnant must use birth control.

Exclusion Criteria:

• Have had a live vaccination within 12 weeks before randomization, or intend to have a live vaccination during the course of the study.
• Have had had recent surgery or are scheduled to have surgery during the study.
• Are immunocompromised or have evidence of active infection (such as hepatitis, tuberculosis or, human immunodeficiency virus [HIV]).
• Have been on certain drugs that are being studied for RRMS or have recently received prescription drugs to treat RRMS.
• Have had a recent serious infection.
• Have serious or uncontrolled illnesses other than RRMS.
• Have clinically significant blood test values.
• Have multiple or severe drug allergies.
• Have contraindications for magnetic resonance imaging (MRI; "scanning") or claustrophobia (fear of an enclosed space) that cannot be managed.