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In an effort to identify the signal compound that mediates systemic acquired resistance (SAR), changes in the content of phloem sap were monitored in cucumber plants inoculated with either tobacco necrosis virus or the fungal pathogen Colletotrichum lagenarium. The concentration of a fluorescent metabolite was observed to increase transiently after inoculation, with a peak reached before SAR was detected. The compound was purified and identified by gas chromatography-mass spectrometry as salicylic acid, a known exogenous inducer of resistance. The data suggest that salicylic acid could function as the endogenous signal in the transmission of SAR in cucumber.