Streptomycin sulphate prevented infection of sunflower seedlings by three races of the downy mildew pathogen Plasmopara halstedii (Farl.) Berlese et de Toni. It inhibited emergence of zoospores from zoosporangia and at high concentrations (500 ppm) was phytotoxic. Affected seedlings were stunted and chlorotic. Penicillin had no observable effect on infection or on appearance of the seedlings.