The Arguello model of cancer metastasis to bone has been used extensively to study breast cancer-induced osteolytic disease. The effects of therapy on skeletal disease and on tumour burden in soft organs are traditionally measured using radiography and/or time-consuming histomorphometry, respectively. The purpose of this study was to develop a sensitive and efficient method for evaluating tumour burden in vivo using MDA-231 cells transduced with the E. coli lacZ gene (MDA-231BAG). Osteolysis was measured by radiography and tumour burden was measured histomorphometrically or biochemically. In untreated mice, measurements of tumour burden in bone extracts using human cytokeratin-associated tissue polypeptide antigen (TPA) ELISA or E. coli beta-galactosidase (beta-gal) activity immunoassay reflected the extent of osteolytic disease as measured by radiography; however, tumour load could be detected before onset of osteolysis. When monitoring the effect of therapy (0.2 mg/kg ibandronate/day), radiography alone proved to be insufficient. Mice treated with the bisphosphonate ibandronate from time of inoculation with cancer cells had no radiologically visible signs of osteolysis but significant tumour load was measured in the bone extracts using these assays. Furthermore, beta-gal activity could be used as a measurement of tumour load in soft organs, and unlike other human breast cancer markers expressed by the MDA-231 cells in vitro, beta-gal activity was detected in the serum of mice with progressive disease. In conclusion, we describe an efficient model of breast cancer-induced osteolysis to quantify the effect of therapy on disease load and distribution, which could be beneficial in evaluating novel therapies for the treatment of the disease.