The growth behaviour of yeast cultures in aqueous suspension was monitored by visible light extinction and showed an exponential growth rate reproducible within ±3% limits. When the cultures were irradiated by c.w. microwave fields of a few mW/cm2 the growth rate either stayed constant or was considerably enhanced or reduced depending on the frequency around 42 GHz. A spectral fine structure with a width of the order of 10 MHz was observed. Careful temperature monitoring excludes a trivial thermal origin of this effect.