The interaction of thioflavin T (ThT) with serum albumins from four different mammalian species i.e. human, bovine, porcine and rabbit, has been investigated by circular dichroism (CD), fluorescence spectroscopy and ITC. The binding constant (K) for HSA was found to be 9.9 × 104 M-1, 4.3 × 104 M-1 for RSA, 1.07 × 104 M-1 for PSA and 0.3 × 104 M-1 for BSA and the number of binding sites (n) were 1.14, 1.06, 0.94 and 0.8, respectively, which is very significant. By using unfolding pathway of HSA in the presence of urea, domain II of HSA has been assigned to possess binding site of ThT. Its binding constant is comparable to many drugs that bind at domain II of HSA, like salicylate, warfarin, digitoxin, etc. Acting force between HSA and ThT is showing that both hydrophobic and electrostatic forces have contributed for the interaction. ΔGbinding, ΔH and ΔS were calculated to be -28.46 kJ mol-1, -3.50 kJ mol-1 and 81.04 J K-1 mol-1, respectively. The data described here will help to increase our understanding about the interaction of ThT with native proteins. The results also indicate that care must be taken while using ThT as a probe for detecting amyloid fibrils. © 2009 Elsevier B.V. All rights reserved.