Abstract In this article the kinetics of the insulin-like growth factor binding protein-3 (IGFBP-3) reaction with its specific antibody immobilised on the inner wall of the reaction tube, and the subsequent binding of the immunocomplex formed with a second 125I-labelled antibody are described. These reactions are used in the immunoradiometric determination of IGFBP-3. Independent variables were analyte and labelled antibody, temperature, viscosity, and the ionic strength of the medium. For the global process mono-exponential kinetics were found to be dependent on the concentrations, such dependence fitting with the models discussed in this paper. Viscosity results clearly indicate its negative influence on the direct reaction rate. Ionic strength shows noticeable but not too relevant effects, which suggests that the variation caused by the glycerol addition is not due to the influence of the dielectric constant of the solutions used. The effect of temperature shows activation parameters similar to the viscous flow energy of water, which suggests diffusion control for the global process.