The effects of the inhibitors of polyamine biosynthesis, 2-difluoromethylornithine (DFMO), (E)-2-(fluoromethyl) dehydroornithine (Δ-MFMO), (K)-2-(fluoromethyl) dehydroornithine methyl ester (A-MFMO. CM3), 2-hydrazinoornithine, (2R,5R)-6-heptyne-2,5-diamine (RR-MAF), 2-difiuorornethylarginine (DFMA), cyclohexylamine (CHA) and methylglyoxal-bis (guanylhydrazone) (MGBG) on infection of barley Leaves (Hordeum vulgare L.) by Erysiphe graminis f.sp. hordei Marchal, were examined. Various concentrations of the inhibitors were sprayed onto barley leaves ev. Golden Promise as post-inoculation treatments. DFMO, MGBG and a combination of the two were also applied as pre-inoculation treatments. Each inhibitor substantially reduced mildew infection. DFMO was as efficient as the more recently developed ornithine analogues. With the exception of the DFMA treatments, post-inoculation sprays were more effective than pre-inoculation ones. When DFMO, MGBG and DFMA were sprayed onto leaves at different times, DFMO and MGBG most effectively controlled mildew when sprayed on the third day after inoculation. DFMA was more efficient as a pre-inoculation treatment. Addition of polyamines to DFMO sprays increased mildew infection, compared to that resulting from the DFMO treatment alone, but infection was less than in the controls.