Numerous studies have demonstrated a correlation between hyaluronan expression and the malignant properties of various kinds of cancer, and inhibition of hyaluronan production causes decreased tumor growth. Hyaluronan oligosaccharides have been shown to inhibit several tumor cell types via disruption of receptor-hyaluronan interaction. However, few studies have addressed hyaluronan with respect to osteosarcoma. In this study, we examined the effects of exogenously added hyaluronan oligosaccharides on tumorigenicity of murine osteosarcoma cells, LM-8, and human osteoblastic osteosarcoma cells, MG-63. Moreover, the critical size of oligomers needed to inhibit malignant properties was defined. Fluorescent hyaluronan oligosaccharides accumulated both on the surface of cells and in the cytoplasm, and this retention was blocked by pretreatment with an anti-CD44 monoclonal antibody. Hyaluronan octasaccharides significantly inhibited cell viability and induced apoptosis as defined by cell proliferation and terminal deoxynucleotidyl transferase dUTP nick-end labeling assays, respectively. Octasaccharides also abrogated functional cell-associated matrices and significantly reduced the retention of endogenous hyaluronan. Further, octasaccharide treatment affected an inhibition of cell motility as well as cell invasiveness. Pretreatment of the cells with anti-CD44 antibody reduced the antitumor effect of the octasaccharides. In vivo, intratumoral injection of hyaluronan octasaccharides reduced the hyaluronan accumulation in local tumors, resulting in significant suppression of the formation of distant lung metastasis. Together these data suggest that hyaluronan oligosaccharides have potent antitumor effects functioning in part by the abrogation of hyaluronan-rich cell-associated matrices.