Senescence marker protein-30 (SMP30), composed of 299 amino acids, has an approximate molecular mass of 32-34 kDa and has a pI 4.9 in charge. The amino acid alignment from various animal species revealed a highly conserved structure. SMP30 has an enzyme activity hydrolyzing sarin, soman, and tabun, known as lethal toxic nerve chemicals. We analyzed the organophosphatase activity of SMP30 using DFP as a substrate. This DFPase activity is revealed in a dose-dependent manner in the presence of magnesium ions. We investigated the intracellular localization of SMP30. It is localized in both the cytoplasm and nucleus. To confirm the presence of SMP30 in the nucleus, we prepared nuclear and cytoplasmic extracts from isolated cultured hepatocytes. Western blotting showed that SMP30 was detected in both extracts. Because the expression is reduced by carbon tetrachloride, one can speculate that the expression is modulated by oxidative stress increased with aging.