ELISAs are available for the detection of Tartrate Resistant Acid Phosphatase 5b (TRACP 5b) in serum and plasma. Current products include kits for human, rat and mouse forms of TRACP 5b. These specific method assays detect TRACP 5b activity freshly liberated from osteoclasts. They can be used to determine the increased bone resorption rate in subjects with primary osteoporosis and renal bone disease, and to predict future fracture risk. The assays can be used for monitoring the efficacy of anti-resorptive treatment. In tumour subjects, elevated TRACP 5b activity indicates spreading of cancer cells to bone. In vitro, serum TRACP 5b activity reflects the number of osteoclasts, and therefore the assays can be conveniently used to determine osteoclast number and bone resorption in osteoclast cultures.