The nature of chromium(III) complexes has been found to show a profound influence in its interaction with collagen. The hydrothermal stability of rat tail tendon (RTT) fibres treated with dimeric, trimeric and tetrameric species of chromium(III) has been found to be 102, 87 and 68 degrees C, while that of native RTT is 62 degrees C. This shows that the efficiency of crosslinking of collagen by chromium(III) species is dimeric > trimeric > tetrameric. This order of stabilisation is again confirmed by cyanogen bromide (CNBr) cleavage of RTT collagen treated with dimeric, trimeric and tetrameric chromium(III) species. CNBr has been found to cleave the collagen treated with tetrameric chromium(III) species extensively. On the other hand, dimer-treated collagen does not undergo any cleavage on CNBr treatment. The equilibrium constants for the reaction of a nucleophile like NCS(-) to the dimeric, trimeric and tetrameric species of chromium(III) have been found to be 15.7+/-0.1, 14.6+/-0.1 and 1.2+/-0.1 M(-1), respectively. These equilibrium constant values reflect the relative thermodynamic stability of the chromium(III) species-nucleophile complex. The low stabilising effect of the tetrameric species can be traced to its low thermodynamic affinity for nucleophiles.