The forces a walker exerts to an oscillating floor differ significantly from those exerted to a fixed floor. This is because of a natural and unconscious gait modification, deriving from the necessity of minimizing energy consumption while maintaining balance. Among other factors, these forces depend on the frequency and amplitude of oscillation of the floor, and this dependency is claimed to be responsible for most of the large amplitude vibrations observed on flexible footbridges. This paper reports the results of an extensive experimental campaign carried out using an instrumented treadmill, and aimed at evaluating the characteristics of the lateral force exerted by pedestrians when walking on a floor driven into a harmonic lateral motion with varying frequency and amplitude. The total measured force is separated into motiondependent or self-excited components proportional to the floor velocity and acceleration, respectively, and an external excitation, which is independent of the floor motion. The variation of the acceleration- and velocity-proportional forces together with the characteristics of the spectra of the external excitation is discussed.