Metastasis to bone is the most frequent cause of breast cancer morbidity and mortality. Osteolytic lesions are formed as a result of cancer-induced increased osteoclast activity. Heparin and heparin-like glycosaminoglycans (HLGAGs), which are commonly used to prevent venous thromboembolism, have previously been shown to prolong survival of cancer patients. The effects of HLGAGs on human osteoclasts were studied in vitro by culturing human osteoclasts on bovine bone slices in the presence of different concentrations of HLGAGs for three days. Multinuclear TRACP-positive osteoclasts were visualized by staining the nuclei with Hoechst and the tartrate-resistant acid phosphatase (TRACP) content of the cells using a leukocyte acid phosphatase kit. Resorption pits were visualized using TRITC-conjugated WGA lectin. The upper images of resorption pits (X 20 magnification) indicate that a high-molecular-weight E. coli K5-derived heparin-like polysaccharide (K5-NSOS) inhibits bone resorption activity of osteoclasts (examples of resorption pits are outlined in blue). K5-NSOS did not affect the amount of TRACP positive osteoclasts, as indicated by the lower images (X 10 magnification). Because K5-NSOS is an antimetastatic and antiresorptive compound with low anticoagulant activity, it is a potential therapeutic agent to prevent and/or treat breast cancer metastases. For further details, please see Pollari and colleagues on page 597 in this issue.