Telechelic polymer chains (hydrophobic end-capped poly(ethylene oxide)) linked to a hydrophobic surface were probed with the atomic force microscope tip by vertical stretching. Under conditions that allowed only a limited number of molecular chains to interact with the hydrophobic atomic force microscope tip, the gradient of the forces was found to be quantized in integer multiples of 540 +/- 30 and 330 +/- 25 mu N/m for polymers of mass 20 000 and 35 000, respectively. The measured force gradient quanta is interpreted as the stiffness of a single polymer chain. This work may shed new light on macromolecules at interfaces involving chain extensions and opens the route for elasticity studies of single macromolecules.