  We have performed microwave photoresistance measurements in high mobility
GaAs/AlGaAs quantum wells and investigated the value of the effective mass.
Surprisingly, the effective mass, obtained from the period of microwave-induced
resistance oscillations, is found to be about 12% lower than the band mass in
GaAs, $\mb$. This finding provides strong evidence for electron-electron
interactions which can be probed by microwave photoresistance in very high
Landau levels. In contrast, the measured magnetoplasmon dispersion revealed an
effective mass which is close to $\mb$, in accord with previous studies.
