We investigated the role of glutathione S-transferases Mu 1 (GSTM1) in acetaminophen (APAP)-induced hepatotoxicity using Gstm1-null mice. A single oral administration of APAP resulted in a marked increase in plasma alanine aminotransferase accompanied by hepatocyte necrosis 24 hr after administration in wild-type mice, but its magnitude was unexpectedly attenuated in Gstm1-null mice. Therefore, it is suggested that Gstm1-null mice are resistant to APAP-induced hepatotoxicity. To examine the mechanism of this resistance in Gstm1-null mice, we measured phosphorylation of c-jun N-terminal kinase (JNK), which mediates the signal of APAP-induced hepatocyte necrosis, by Western blot analysis 2 and 6 hr after APAP administration. A marked increase in phosphorylated JNK was observed in wild-type mice, but the increase was markedly suppressed in Gstm1-null mice. Therefore, it is suggested that suppressed phosphorylation of JNK may be a main mechanism of the resistance to APAP-induced hepatotoxicity in Gstm1-null mice, although other possibilities of the mechanism cannot be eliminated. Additionally, phosphorylation of glycogen synthase kinase-3β and mitogen-activated protein kinase kinase 4, which are upstream kinases of JNK in APAP-induced hepatotoxicity, were also suppressed in Gstm1-null mice. A decrease in liver total glutathione 2 hr after APAP administration, which is an indicator for exposure to N-acetyl-p-benzoquinoneimine, the reactive metabolite of APAP, were similar in wild-type and Gstm1-null mice. In conclusion, Gstm1-null mice are considered to be resistant to APAP-induced hepatotoxicity perhaps by the suppression of JNK phosphorylation. This study indicates the novel role of GSTM1 as a factor mediating the cellular signal for APAP-induced hepatotoxicity.