The thyroid hormones T₃ and T₄ translocate the plasma membrane through multifunctional transporter proteins (Hennemann et al, 2001). In liver cells, the hormones may bind Triton X-100 soluble receptor proteins in the plasma membrane that channel T₃/T₄to their transporters (Kemp & Taylor, 1997). A novel approach to studying this type of interaction is surface plasmon resonance (SPR) in which one member of a binding pair is immobilised to a sensor surface and the second member is passed over the immobilised partner. Binding is detected in real time as a change in refractive index at the sensor surface and is proportional to the change in mass due to binding.Sinusoidal membranes purified from livers of humanely-killed rats (Kemp & Taylor, 1997) were immobilised on an L1 sensor chip placed in a Biacore 3000 SPR system (Biacore AB, Uppsala, Sweden). Micromolar concentrations of T₃,T₄ and rT₃ in phosphate-buffered saline were injected over the immobilised membranes. SPR sensorgrams were double referenced with subtraction of non-specific binding to an unmodified sensor surface. Concentration dependent binding of T₃ and T₄ to the membranes was observed and analysed by a 1:1 Langmuir adsorption model using BioEvaluation software. For T₃; ka = 1.51 (±0.01) x 10⁴ Ms^-1, kd = 0.029 (±0.004) s^-1, KD = 1.89 µM. For T₄; ka = 1.36 (± 0.16) x 10⁴ Ms^-1, kd = 0.171 (±0.015) s^-1, KD = 12.6 µM. Here ka and kd represent the on/off rate constants of the interaction while KD is the equilibrium dissociation constant. rTdid not interact with liver membranes.T₃ – receptor interactions were also studied by injecting liver membranes over T₃ immobilised to a CM5 sensor surface (0.8 pmol.mm^-2); 15 µg membrane protein produced a maximum specific binding response (Figure 1). Membranes briefly treated with 0.5% Triton X-100 to solubilise T₃ receptors without loss of membrane vesicular integrity (Kemp & Taylor, 1997) showed a 98% reduction in binding. Crude Triton-extractable membrane protein displayed binding activity towards T₃ bound to the CM5 chip (see Figure 1). These results provide valuable kinetic information on real-time binding interactions of T₃ and T4 with components of liver sinusoidal membrane. This work has demonstrated that SPR is of value in the identification and kinetic characterisation of physiologically important receptor – ligand binding interactions.