Lipid peroxidation has been observed in the interaction of fungi with several groups of unrelated fungicides. However, the primary events that initiate these reactions appear to be quite different from each other and are thus far poorly understood. The stimulation of lipid peroxidation is not a general effect of fungicide action and is specific to dicarboximide fungicides (DCOFs) and aromatic hydrocarbon fungicides (AHFs). Other antifungal inhibitors such as aldimorph, tridemorph, dichlo-fluanid, and metalaxyl had no effects on lipid peroxidations at concentrations sufficient to inhibit growth. The induction of lipid peroxidation by DCOFs and AHFs is most likely not caused via a direct conversion of these fungicides to free organic radicals. A prerequisite for the peroxidation of polyunsaturated fatty acids in living cells is either the presence of an active enzyme, or a more general oxidative stress situation caused by free radicals or by reduced oxygen specimens.