The eradicant effect of a number of fungicides on potato late blight development was observed under laboratory and field conditions. M.C.O., Dithane M-22 and the antibiotic dihydrostreptomycin were found to reduce necrosis of the leaf significantly when used within 12 to 24 hours after inoculation with the fungus, and to significantly reduce sporulation when used within 12 to 48 hours. The number of blight lesions developing per plant was significantly reduced by treating field plots with M.C.O. 17 hours after inoculation with the fungus. Dithane M-22 was not as effective as M.C.O. in reducing lesion development but was found to have a marked inhibitory effect on sporulation.