Human serum tartrate-resistant acid phosphatase exists as two enzyme isoforms (TRACP 5a and 5b), derived by differential, post-translational processing of a common gene product. Serum TRACP 5b is from bone-resorbing osteoclasts (OC) and becomes elevated in diseases of increased bone resorption. TRACP 5a is secreted by macrophages (MPhi) and dendritic cells (DC) and is increased in many patients with rheumatoid arthritis. Our purpose was to fully characterize the properties of human TRACP isoforms and to produce an antibody specific to TRACP 5a for use as a biomarker in chronic inflammatory diseases. Partially purified, natural serum TRACP isoforms and recombinant TRACP 5a (rTRACP 5a) were compared with respect to specific activity and subunit structure and presence of sialic acid. Mice were immunized with rTRACP 5a, and resulting hybridomas were screened for monoclonal antibody to serum TRACP 5a. One antibody, 220, was tested for its epitope specificity and use in various immunological techniques. rTRACP 5a had properties identical to serum TRACP 5a. Antibody 220 was specific for the trypsin-sensitive epitope in the loop peptide, present only in TRACP 5a. Antibody 220 was effective for specific immunoprecipitation, immunoassay, and immunoblot of TRACP 5a. Intact TRACP was present in MPhi, DC, and OC. TRACP 5a was the predominant isoform secreted by MPhi and DC, whereas TRACP 5b was the predominant isoform secreted by OC. TRACP isoforms 5a and 5b may have different functions inside and outside of monocyte-derived cells. Antibody 220 is an important resource for studies of the biosynthetic relationship among TRACP isoforms and of the significance of serum TRACP 5a as a marker in diseases of bone metabolism and inflammation.