The enzyme ornithine decarboxylase (ODC) from Botrytis cinerea has been purified 68-fold with 74% recovery. The optimum pH was 7.3 and the Km for ornithine was 70μM. Using this preparation, the effects of several potential enzyme-activated inhibitors of ODC have been studied, comparing their properties with those of difluoromethylornithine (DFMO) which is a known growth inhibitor for Botrytis and other fungi. Difluoromethylarginine and dehydromonofluoromethylarginine, which are at least as effective as DFMO for the inhibition of growth of Botrytis cinerea, are relatively poor inhibitors of ODC. R,R-Methylacetylenicputrescine and dehydromonofluoromethylornithine, which are relatively poor inhibitors of Botrytis growth, are very efficient inhibitors of the ODC. © 1982.