The papain superfamily member bleomycin hydrolase (Blmh) is a neutral cysteine protease with structural similarity to a 20S proteasome. Bleomycin (BLM), a clinically used glycopeptide anticancer agent, is deaminated in vitro by Blmh. We used gene targeting to generate mice that lack Blmh and demonstrated that Blmh is the sole enzyme required for BLM deamination. Although some Blmh null mice were viable and reproduced, only about 65% of the expected number survived the neonatal period, revealing an important role for Blmh in neonatal survival. Mice lacking Blmh exhibited variably penetrant tail dermatitis that resembled rodent ringtail. The histopathology of the tail dermatitis was similar to skin lesions in humans with pellagra, necrolytic migratory erythema, and acrodermatitis enteropathica. Compared with controls, Blmh null mice were more sensitive to acute BLM lethality and developed pulmonary fibrosis more readily following BLM treatment. Thus, we have established that Blmh is an essential protectant against BLM-induced death and has an important role in neonatal survival and in maintaining epidermal integrity.