Tissue deposits of basic calcium phosphate (BCP) crystals are associated with various clinical manifestations of inflammation. We addressed the possibility that native proteins modify the ability of hydroxyapatite (HA) crystals to stimulate human inflammatory cells. Neutrophil superoxide release and chemiluminescence in response to HA crystals (0.3–4.0 mg/ml) were blunted by serum and plasma. Inhibitory activity was progressively removed from serum by sequential adsorption with HA crystals, suggesting that the inhibitors were crystal‐bound proteins. Thus, we characterized HA crystal‐bound plasma proteins by O'Farrell gels: Fibronectin, transferrin, albumin, α₂−HS glycoprotein (AHSG), α₁−proteinase inhibitor, α₁−acid glycoprotein, Gc globulin, haptoglobin, and high density lipoprotein apolipoproteins were major bound species. Of these, AHSG was the most active inhibitor of HA‐induced neutrophil superoxide release, and this glycoprotein partially (60%) restored inhibitory activity to HA‐adsorbed serum. AHSG also bound in vitro to the related BCP crystal, octacalcium phosphate, but only minimally to calcium pyrophosphate dihydrate crystals and monosodium urate crystals. Suppressive effects on neutrophil stimulation exhibited by AHSG were also specific for BCP crystals. AHSG was present in noninflammatory synovial fluids bound to synthetic HA crystals in vitro, and AHSG could be detected on native synovial fluid HA crystals. We conclude that the binding of AHSG may modulate the inflammatory potential of BCP crystals.