Heparanase promotes bone-to-bone metastasis. Six weeks after injection of heparanase-transfected or control-transfected CAG myeloma cells into the tibia of SCID mice, sera were harvested and analyzed for the level of human immunoglobulin kappa light chain as a measure of tumor burden. Plots show the kappa light chain level for each individual mouse. The mean kappa chain level for the heparanase group was significantly higher than that of the control group (heparanase group, 6.13 μg/mL ± 1.33 μg/mL, control group 2.36 μg/mL ± 1.02 μg/mL [P = .04]), although several animals within the heparanase group had low tumor burden. Values are means plus or minus SEM. Metastases of tumor to distant bone were assessed by immunostaining for the presence of kappa light chain in the femur contralateral to the injected tibia. • denotes animals with bone metastases; ○, those without detectable bone metastases.