We present medium resolution HST FOS spectra of the ionizing star clusters in four giant H II regions in the spiral galaxy M 101, spanning a range in [O/H] from 8.2 to 8.9. The observed energy distributions in the wavelength range 1150 Angstrom to 8200 Angstrom are compared to population synthesis models of extremely young star clusters, taking into account the attenuation by dust and the nebular continuum emission. The OB cluster spectra are compatible with short duration, 3 Myr old star formation bursts governed by a Miller-Scale type solar neighborhood initial mass function (IMF). The attenuation law towards the targets in M 101 is found to be rather similar to the average galactic law but for a much weaker 2175 Angstrom bump. Since the observed shape of the composite continua is dominated by the form of the UV extinction, power-law IMFs skewed towards high stellar masses may also explain the observations with slightly modified extinction curves. However, on the basis of the data presented here there is no need to invoke second order effects such as a variation with metallicity of the slope of the IMF, or the upper and lower stellar mass limits in the current burst of star formation respectively.