A Study to Investigate the Safety, Tolerability, and Processing by the Body of Intravenous and Subcutaneous RO7121932 in Participants With Multiple Sclerosis.

Inclusion Criteria:

• Expanded Disability Status Scale (EDSS) score ≤7.0 at Screening

• Participants with relapsing multiple sclerosis (RMS) or progressive multiplesclerosis (PMS) who fulfil international panel criteria for diagnosis (McDonald 2017criteria)

• Participants not treated with any approved MS treatment at Screening and notplanning to start on any MS therapy during the study (including follow-up)

• Female participants must practice abstinence or otherwise use contraception

Exclusion Criteria:

• Evidence of clinical disease activity as defined by any clinical relapse within 3months prior to screening, or by >1 clinical relapse within 12 months prior toscreening

• Evidence of magnetic resonance imaging (MRI) activity as defined by the presence of ≥ 1 Gadolinium-enhancing T1 lesion in the screening MRI scan or by ≥ 4 new orenlarging T2 lesions in the screening scan as compared to a reference scan

• Participants who have active progressive multifocal leukoencephalopathy (PML), havehad confirmed PML, or have a high degree of suspicion for PML

• Known presence of other neurological disorders that may mimic MS including but notlimited to: neuromyelitis optica spectrum disease, Lyme disease, untreated VitaminB12 deficiency, neurosarcoidosis, cerebrovascular disorders, and untreatedhypothyroidism

• Known active or uncontrolled bacterial, viral, fungal, mycobacterial infection orother infection, excluding fungal infection of nail beds, including participantsexhibiting symptoms consistent with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) within 6 weeks prior to Day 1

• Participants with a current diagnosis of epilepsy

• Clinically significant cardiac, metabolic, hematologic, hepatic, immunologic,urologic, endocrinologic, neurologic, pulmonary, psychiatric, dermatologic,allergic, renal, or other major diseases

• History of cancer, including hematologic malignancy and solid tumors, within 10years of screening. Basal or squamous cell carcinoma of the skin that has beenexcised and is considered cured and in situ carcinoma of the cervix treated withapparent success by curative therapy >1 year prior to screening is not exclusionary

• Any concomitant disease that may require treatment with systemic corticosteroids orimmunosuppressants during course of the study

• History of currently active primary or secondary (non-drug-related) immunodeficiency

• History of hypersensitivity to biologic agents or any of the excipients in theformulation

• Only for cohorts where CSF samples are planned to be collected: Participants with ahistory of spinal cord compression, raised intra-cerebral pressure, clinicallysignificant vertebral joint pathology or any other current abnormalities in thelumbar region which could prevent the lumbar puncture procedure.

Prior/Concomitant Therapy:

• Treatment with any approved MS treatment at Screening. Participants may becomeeligible after completion of a washout period prior to acquiring any screeninglaboratory tests but should not be withdrawn from therapies for the sole purpose ofmeeting eligibility for the trial

• Previous treatment with RO7121932, alemtuzumab, cladribine, mitoxantrone,cyclophosphamide, total body irradiation, bone marrow transplantation, andhematopoietic stem cell transplantation. For the USA only, previous treatment withdaclizumab

• Previous treatment with anti-CD20 B-cell-depleting therapies (e.g., rituximab,ocrelizumab, or ofatumumab)

• <12 months prior to acquiring any screening laboratory tests,

• ≥12 months prior to acquiring any screening laboratory tests, if B-cells areoutside the normal range, or not back to individual baseline ± 20% (if data areavailable),

• If discontinuation of a prior B-cell depletion therapy was motivated by safetyreasons

• Current or prior treatment with natalizumab (if <24 months prior to acquiring anyscreening laboratory tests)

Prior/Concurrent Clinical Study Experience:

• Participation in an investigational drug medicinal product or medical device studywithin 30 days before Screening or within five times the pharmacodynamic (PD) orpharmacokinetic (PK) half-life (if known), whichever is longer

Diagnostic Assessments:

• Positive result on human immunodeficiency virus (HIV1) and HIV2, hepatitis C, orhepatitis B

• Participants with suicidal ideation or behavior within 6 months prior to Screeningor participants who, in the Investigator's judgment, pose a suicidal or homicidalrisk

• Vaccination with a live or live-attenuated vaccine within 6 weeks prior to Day 1