Previous investigations have indicated a deleterious leakage of Ti(III) and/or Ti(IV) species from Ti-Al-V alloy joint prostheses into adjacent tissue, synovium or synovial fluid (SF) in vivo. In view of the importance of the particular chemical nature of such complexes in determining their biological activity, we have employed high field proton (H-1) NMR spectroscopy to "speciate" Ti(IV) in inflammatory SF. Treatment of osteoarthritic SF samples with increasing concentrations of Ti(IV) (0.10-1.03mM [TiO(C2O4)(2)](2-)) gave rise to a specific broadening of the citrate proton resonances, indicating that this bioavailable oxygen-donor ligand plays an important role in complexing implant-derived Ti(IV). H-1 NMR analysis of Ti(IV)-loaded SF samples subsequently treated with a large excess of ascorbate (0.05 M) showed that this added Ti(IV) chelator was only poorly effective in removing this metal ion from Ti(IV)-citrate/Ti(IV)-oxycitrate complexes. The results obtained here provide evidence for complexation of the low-molecular-mass (non-protein-bound) fraction of implant-derived Ti(IV) by citrate in vivo. (c) 2005 Elsevier Inc. All rights reserved.