Non-covalent bonds of the same type as antibody-antigen bonds--"affinity bonds"--are one of the most important types of structural bonds on the cellular level, mediating interactions such as cell-cell and cell-extracellular matrix adhesion, providing the integrity of such structures as microtubules and actin filaments, and producing force in muscle, flagella and cilia. The affinity bonds in these and many other interactions are subject to forces during the life of the cell. However, the tensile strength of these bonds, and how this determines the strength of the interactions on the cellular level, is largely unknown. We have attempted to calculate the tensile strength of an affinity bond by modelling binding as a multistep process in which the binding molecules first diffuse together, overcome an activation energy, and then bind. By calculating the energies associated with each step, the energy actually contained in the bond can be determined, yielding the bond strength. The "average" affinity bond is thus found to have a tensile strength of about 40 mu dyn. However, the temporal nature of affinity bonds (thermal fluctuations will eventually break them) causes the strength of the interactions to be actually somewhat weaker, with the outcome of putting tension on a group of bonds depending strongly on the conditions.