Calcium pyrophosphate dihydrate (CPPD) crystals are common components of osteoarthritic joints and correlate with a poor prognosis. Transglutaminase (Tgase) enzymes have been implicated in pathologic mineralization in cartilage; yet, definitive studies linking Tgase activity to CPPD crystal formation in osteoarthritic articular cartilage are lacking. We measured in-vivo Tgase activity in osteoarthritic and normal human cartilage, and explored the effect of Tgase inhibitors on CPPD crystal formation by normal chondrocytes. Osteoarthritic articular cartilage from was obtained from specimens discarded at the time of knee replacement surgery. Normal adult cartilage samples from a tissue bank were used as controls. Tgase-specific isopeptide (epsilon-(gamma-glutamyl) lysine) bonds were measured in cartilage extracts by HPLC. Tgase-specific crosslinks were localized in osteoarthritic cartilage by immunohistochemistry. The effect of Tgase inhibition was determined in an in-vitro model of CPPD crystal formation. Tgase-specific crosslink levels were 1.55 +/- 0.3 picomoles/ng protein in normal human adult articular cartilage and 4.74 +/- 0.7 picomoles/ng protein in osteoarthritic human cartilage (p less than 0.001). Immunostaining confirmed the presence of Tgase crosslinks in the pericellular matrix of chondrocytes at potential sites of CPPD crystal formation. Tgase inhibitors suppressed CPPD crystal formation by porcine chondrocytes. These findings support a role for Tgase in CPPD crystal formation in aging or degenerated cartilage.