Non-specific protein immobilisation at the liquid|solid interface (of a microtiter plate) is one of the most commonly used methods of preparing wells for ELISA. This immobilised target is recognized by soluble molecules like antibodies or peptide-displaying phages. Stages of ELISA, which assumes specific recognition, may be described using a modified Langmuir-like isotherm . It should be noted that this model cannot be applied for initial non-specific precaution . Since wells are coated with TnT molecules and the number of available target molecules (active sites), they may bind a limited number of phages. Moreover, the number of bound phages limits the amount of bound HRP-conjugated antibodies. The surface density of HRP enzyme (i.e., the amount of HRP immobilised on the liquid|solid interface) is a crucial factor affecting the rate of substrate (3,3',5,5'-tetramethylbenzidine, TMB) oxidation. The reaction rate may be expressed by the rate of the change of absorbance at 370 nm (which is the maximum of absorption of yellow by-product ) under the assumption that the kinetics of the enzymatic reaction is zero order (Michaelis-Menten model and enzyme saturation). In this case, the measured signal is the enzyme activity defined as the slope of the linear time function of absorbance (Eq.13). Enzyme activity is proportional to the amount of bound HRP-conjugated antibodies and indirectly to the number of bound phages (Eq.14). When the reaction is stopped by addition of sulfuric acid, the enzyme is denatured. As a result, the blue complex dissociates into a yellow product (adsorption maximum at 450 nm) and transparent TMB .