The effect of different plasma parameters on the stability of the half-harmonic electron gyrofrequency waves is investigated. By making some simplifying assumptions about the cold plasma and using a ring distribution for the warm component the role of the different parameters is demonstrated. A maximum value of the ratio of the cold to hot plasma density above which we cannot get instability is derived, and this ratio increases with the temperature anisotropy. Calculations of the growth rates show that there is a sharp peak when ϑ=tan-1 k⊥/k∥~70 ¿-80 ¿.