Anti-transpiration and anti-growth activities of xylem sap were studied for maize, sunflower, cotton and castor bean, which were subjected to several stress treatments: soil drying, flooding and salinity. All samples of xylem sap showed an increased concentration of proteins when plants were either soil-dried, salt-treated or flooded. The protein transportation flux in xylem sap, i.e. the product of protein concentration and volume flux, was also increased as a result. Electrophoresis using SDS–PAGE analysis showed that xylem proteins were of variable size. When samples of xylem sap were used to feed detached leaves or shoots in a bioassay system for their anti-transpiration and anti-growth activities, such xylem proteins in their original concentrations apparently showed neither direct inhibition nor enhanced effects of ABA. All samples of xylem sap from plants subjected to different stresses showed anti-transpiration and anti-growth activities that could be fully accounted for by their ABA concentration. In cases where such activities were more than could be explained by the ABA concentrations, filtration with a 0·20 μm microfilter removed the discrepancies.