In this paper the velocity distribution functions (VDFs) of protons measured by Helios in fast solar wind are analyzed in the framework of quasilinear theory (QLT). Evidence is presented that the shape of the central isodensity contours in velocity space and the temperature anisotropy of the core part of the VDFs can be explained by wave-induced plateau formation according to QLT. The plateau is formed by protons that are in resonance with cyclotron waves, which are assumed to propagate both outwardly and inwardly at phase speeds following from the plasma dispersion relation. For the proton VDFs measured near 0.3 AU in fast low-beta solar wind, the theoretical predictions of QLT, using the cold plasma dispersion relation, agree well with the in situ observations. For the proton VDFs measured near 1 AU in fast high-beta wind, the predictions of QLT, using again the cold plasma dispersion relation, only give an upper limit for the anisotropy. Yet, considering thermal effects in the dispersion relation, a better agreement between the theory based on resonant ion diffusion and the observations is obtained. For nondispersive waves a simple relation between the ion thermal speed parallel to the magnetic field and the ion-temperature anisotropy is derived, which is shown to be consistent with the anisotropy of the heavy O+5 ion as observed on the Solar and Heliospheric Observatory (SOHO), as well as with the anisotropy predicted numerically by a hybrid simulation of the ion-temperature regulation by waves.