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

Inclusion Criteria:

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

Exclusion Criteria:

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