An inhibitor of heparanase activity inhibits myeloma tumor growth in vivo. Established subcutaneous tumors formed by CAG myeloma cells expressing heparanase at either high (HPSE^high) or low (HPSE^low) levels were treated for 28 days with PBS or ¹⁰⁰NA,RO-H at doses of 18 mg/kg per day (both HPSE^high and HPSE^low cells) or 36 mg/kg per day (HPSE^high cells only). At termination of the treatment period, tumors were harvested and photographed. X indicates no gross tumor detectable in the animal. Tumor wet weights and P values are shown in Tables 1 and 2.