Motivated by the need to explain observed elliptically polarized emiss
ion from Jupiter, the mechanism of electron-cyclotron maser emission i
s considered for drifting electron distributions, where the electrons
stream with a non-zero mean velocity parallel to the magnetic field li
nes. An analytical expression for the semirelativistic growth rate is
derived and its properties analysed in detail for waves generated in t
he magneto-ionic modes. The main features of the growth rate are discu
ssed, on the basis of a geometric analysis using resonant ellipses.