The existence of a microwave effect on the growth of yeastcells was demonstrated. The mechanismen of this resonance-like effect is not clear. The resonancefrequency remains the same when the external intensity applied is changed. There is a clear indication of strong negative effects on both sides of the resonancefrequency (although extremely low intensities are used). For the first time a narrowing of the resonance width is observed when irradiation of lower intensities is applied. This result shows that the half-width of a resonance is intensity dependent.