6(5H)-Phenanthridinone is an inhibitor of poly(ADP-ribose) polymerase 1 (PARP1) and PARP2. It decreases radiation-induced PARP activity and proliferation of RDM4 murine lymphoma cells. 6(5H)-Phenanthridinone reduces NF-κB-induced transcription of the genes encoding TNF-α, IL-2, and IFN-γ in rat lymphocytes. In vivo, 6(5H)-phenanthridinone reduces spinal cord expression of inducible nitric oxide synthase (iNOS), IL-1β, TNF-α, IL-2, and IFN-γ and reduces disease score in a rat model of experimental autoimmune encephalomyelitis (EAE). It also decreases serum levels of lactate dehydrogenase as well as hepatic lipid peroxidation, oxidative DNA damage, and PARP levels.