Many therapeutic monoclonal antibodies (mAbs) are in development for a wide array of indications. Because of the nature of the product and/or the intended pharmacology, mAbs are assessed for their potential immunomodulation, whether intended or not. Immunomodulation and immunotoxicology (i.e., immunosuppression, immunogenicity, immunostimulation) can be related. The intended immunomodulatory effect of mAbs is the intended pharmacology for clinical efficacy. Exaggerated or prolonged pharmacological activity may, however, result in adverse immunomodulation, leading to unpredictable and unintended immunotoxicology outcomes. Nonhuman primates (NHPs) are often considered as the most relevant species to perform preclinical studies for the testing of mAb-induced immunomodulation and immunotoxicity. This chapter focuses on regulatory considerations associated with the testing of mAbs in NHPs, the mAb product and biomanufacturing process characteristics to take into consideration for immunogenicity testing, and the preclinical study designs in adult and juvenile NHPs to monitor immunomodulation.