Human serum contains 2 isoforms of type-5 tartrate-resistant acid phosphatase (TRACP): 5a and 5b. TRACP-5b is osteoclastic. Our goal was to determine if serum TRACP-5a could originate from inflammatory macrophages (MPhi). We stained 246 paraffin-embedded tissue samples for TRACP using monoclonal antibody 9C5 (mab9C5) to isoforms 5a and 5b and a novel mab220 specific to isoform 5a. CD68 and lysozyme were also stained. MPhi of chronic and granulomatous inflammation and in tissues that undergo strong antigenic stimulation were strongly positive for TRACP, more so with mab220 than with mab9C5. Noninflammatory MPhi in lymph node sinuses or germinal centers and red pulp MPhi of spleen were weak or negative for TRACP. Marginal zone lymphocytes and sebaceous glands of skin were weakly positive for TRACP. Tissue mast cells displayed strong TRACP staining. Neuroendocrine cells of gastrointestinal tissues were strongly immunoreactive with mab9C5 but negative with mab220. Restricted expression of TRACP primarily in inflammatory MPhi supports our hypothesis that circulating TRACP-5a could be a biomarker of chronic inflammatory disease activity.