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Transgenic potato and tobacco plants carrying the osmotin gene under the control of the cauliflower mosaic virus 35S promoter constitutively overexpressed osmotin to a level of approximately 2% of total cellular protein. Leaves of transgenic potato plants exhibited delayed development of disease symptoms after inoculation with spore suspensions of Phytophthora infestans, which is the cause of late blight disease of potato. In contrast, transgenic tobacco plants did not display any change in the development of disease symptoms when challenged with either spore suspensions or fungal mycelia of Phytophthora parasitica var. nicotianae. Using in vitro assays, purified osmotin was found to be more effective against P. infestans. Some inhibition of P. parasitica also was observed in vitro even though no in vivo effect could be established.