Hyaluronan is a major glycosaminoglycan component of the extracellular matrix and CD44 is its principal ligand. In previous in vitro studies we have shown that CD44 and hyaluronan are involved in the invasive properties of the human breast cancer cell line Hs578T. The aim of this study was to test whether experimental therapy with hyaluronan interferes with tumor invasion and has an inhibitory effect on tumor growth in vivo. The Hs578T cell line was xenotransplanted orthotopically into the mammary fat pad of nu/nu mice. After tumor growth reached a maximum size of 5 x 5 mm, 50 microg of hyaluronan was injected intratumorally. The tumors of control nu/nu mice were injected with PBS. Four of 12 tumors from the hyaluronan-treated group regressed completely. This effect could be due to a saturation of the hyaluronan-binding sites on tumor cells or to an acceleration of tumor rejection by a non-T-cell-dependent mechanism. This study gives a rationale for future work on the antineoplastic effects of hyaluronan.