Tartrate-resistant acid phosphatase (TRACP) is a biomarker with multiple clinical and basic applications. It has long been used as a cytochemical marker to identify hairy cell leukemia cells and as a marker for differentiated cells of monocyte lineage including macrophages (MΦ), osteoclasts (OC), and dendritic cells (DC). The clinically relevant MΦ/OC/DC TRACP is called Type-5 acid phosphatase encoded by the Acp5 gene. TRACP consists of two isoforms (Type 5a and 5b) that have different biochemical properties, clinical significance and perhaps biological functions. TRACP5a circulates as a 35 kDa glycoprotein with an intact central regulatory loop peptide, which interacts with the active site. TRACP5b circulates as a disulfide linked heterodimer in which the loop peptide has been proteolytically processed to yield 16 kDa and 23 kDa fragments. Osteoclasts secrete TRACP5b during bone resorption making it a sensitive and specific serum biomarker for OC number and bone resorption rate. TRACP5a, but not 5b, is secreted by activated macrophages and dendritic cells. Elevated serum TRACP5a is emerging as a potential marker for systemic macrophages and chronic inflammation. The principal clinical application of serum TRACP immunoassay is to estimate isoform 5b activity as a marker of osteoclast number and bone resorption rate. This is done in commercial immunoassays by increasing the pH to a level selectively permissive for isoform 5b activity and / or by using a substrate selective for isoform 5b. The antibody used in this ELISA reacts with a critical epitope and binding to TRACP protein causes inactivation of the enzymatic activity.