A Clinical Study to Evaluate the Effects of RO7875913 in Healthy Participants and of RO7875913 With Cevostamab in Participants With Relapsed/Refractory Multiple Myeloma

General:

• Agreement to adhere to the contraception requirements

Part A:

• Body weight > 40 kilogram (kg) with a body mass index of 18-30 kg per meter square (kg/m^2)

Part B:

• Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1
• Life expectancy of at least 12 weeks
• Agreement to provide bone marrow biopsy and aspirate samples

General:

• Treatment with any vaccine within 4 weeks prior to initiation of study drug, or vaccination scheduled to occur during the study
• History or current cardiovascular or pulmonary disease that may limit the ability to respond to systemic infusion/injection reactions
• Positive test result for hepatitis B surface antigen, hepatitis C virus (HCV), or human immunodeficiency virus (HIV) antibody screen
• History of any malignancy
• Major surgical procedure within 28 days prior to initiation of study treatment, or anticipation of need for a major surgical procedure during the study
• History or clinical manifestations of significant metabolic, hepatic, renal, pulmonary, cardiovascular, hematologic, gastrointestinal, urologic, neurologic, or psychiatric disorders
• Known allergy or hypersensitivity to any component of the RO7875913 formulation

Part A:

• Treatment with investigational biologic therapy (or blinded comparator) within 90 days or 5 drug elimination half-lives, whichever is longer, prior to initiation of study drug
• Treatment with investigational non-biologic therapy (or blinded comparator) within 28 days or 5 drug elimination half-lives, whichever is longer, prior to initiation of study drug
• Clinically apparent or familial history of autoimmune disease

Part B:

• Treatment with any systemic chemotherapeutic agent, or treatment with any other anti-cancer agent (investigational or otherwise) within 4 weeks or 5 half-lives of the drug, whichever is shorter, prior to first dose of study treatment
• Treatment with any immunosuppressive medication within 2 weeks prior to first dose of study treatment
• Absolute plasma cell count exceeding 500/mL or 5% of the peripheral blood white cells