Arenaviruses, such as Lassa virus (LASV) and Pichindé virus (PICV), are enveloped viruses with a bi-segmented ambisense RNA genome. The large (L) genomic segment encodes the Z matrix protein and the L RNA-dependent RNA polymerase, whereas the small (S) genomic segment encodes the nucleoprotein (NP) and the glycoprotein (GPC). The NP encapsidates viral genome, is required for viral transcription and replication, and acts as a type I interferon (IFN) antagonist. This article describes assays to demonstrate that NP contains 3′-5′ exoribonuclease (RNase) activity to degrade modeled RNA of the pathogen-associated molecular pattern type and suppresses the IFNβ promoter-driven luciferase reporter gene. The minigenomic (MG) assay is used to assess the role of NP in replicating and transcribing a viral promoter-driven luciferase reporter gene. These powerful assays demonstrate the versatility of NP in mediating viral replication as well as in modulating host innate immune responses.