The effect of griseofulvin on lipid constituents and membrane permeability ofMicrosporum gypseum has been investigated. Mycelia grown in medium containing griseofulvin (IC50 concentration) possessed a lower content of total lipids, phospholipids and sterols. This inhibitory effect was further supported by decreased incorporation of [14C] acetate in total lipids, total phospholipids and sterols. Decrease in total phospholipids was also reflected to a varying extent in all individual phospholipids. An increase in the unsaturated to saturated fatty acid ratio was observed in mycelia grown in medium containing griseofulvin. Membrane permeability was affected by griseofulvin as shown by increased K+-efflux and greater leakage of intracellular [32P] labelled components from prelabelled cells. Our results suggest that the antifungal activity of griseofulvin is partially due to its secondary effect on lipid constituents ofMicrosporum gypseum.