4,4'-Oxybis(benzenesulfonohydrazide) (OBSH) is a blowing agent widely used in the manufacture of porous plastics and rubber. OBSH was notified as an additive in the Japanese positive list system for food utensils, containers and packaging. The in vitro mutagenicity of OBSH was shown extensively in bacterial reverse mutation assays, a DNA repair test, and a chromosomal aberration test. Few studies exist on in vivo genotoxic evaluation on OBSH apart from an in vivo micronuclei test. To clarify in vivo mutagenicity, we conducted a transgenic rodent gene mutation (TGR) assay (OECD TG 488). We dosed male Muta^TM Mouse with OBSH by oral gavage at 0 (negative control), 25, 50, and 100 mg/kg/day for 28 consecutive days, and evaluated mutant frequencies (MFs) of lacZ in the liver and glandular stomach (5 mice/group). We observed two deaths and a reduction in body weight at 100 mg/kg/day. Although we exposed Muta^TM Mouse to OBSH orally for 28 days up to the maximum tolerated dose, we did not detect in vivo mutagenicity in the liver and glandular stomach. In contrast, in the positive control we detected significantly increased MFs. The results of this study suggest that OBSH is not mutagenic in vivo.