Chitosan oligosaccharides(COS)is derived from the chemical or enzymatic decomposition of chitosan.The exact mechanisms underlying many biological functions of COS have not been elucidated.Since subcellular localization is very important in determining biological activity in a number of molecules,we used fluorescein isothiocyanate-labeled chitosan oligosaccharides(FITCCOS)and investigated the localization of COS in living cells by laser scanning microscopy.We found that COS entered cells in a dose-and time-dependent manner,and we first showed that COS may enter cells by facilitated passive diffusion and was preferentially localized in the mitochondria.While in high concentration,it was also found in the cytoplasm and nucleus and was enriched in the nucleolus and karyotheca.The different subcellular localization of COS suggested that they played different effects.Determination of COS subcellular localization in cells is important to help us understand the potent mechanisms underlying its multiple functions. Chitosan oligosaccharides (COS) is derived from the chemical or enzymatic decomposition of chitosan. The exact mechanisms underlying many biological functions of COS have not been elucidated. Since subcellular localization is very important in determining biological activity in a number of molecules, we used fluorescein isothiocyanate -labeled chitosan oligosaccharides (FITCCOS) and investigated the localization of COS in living cells by laser scanning microscopy. We found that COS entered cells in a dose-and time-dependent manner, and we first showed that COS may enter cells by facilitated passive diffusion and was preferentially localized in the mitochondria. Whilst in high concentration, it was also found in the cytoplasm and nucleus and was enriched in the nucleolus and karyotheca.The different subcellular localization of COS suggested that they played different effects. Determination of COS subcellular localization in cells is important to help us understand the potent mechanisms underlying its multiple functions.