SUMMARYKinetin supplied to the lower surfaces of petunia leaf strips reduced the number and size of local lesions following mechanical inoculation of the upper surfaces with tomato spotted‐wilt virus' (TSWV). Glucose increased lesion production, but glucose with kinetin reduced it. Kinetin mixed with infective sap did not affect lesion production.Lesions were increased with increasing water content of leaf strip and the regression equation for the relationship is given. Treatment with kinetin after inoculation reduced lesion production over a wide range of water contents, but was most effective at high values.Kinetin inhibited virus multiplication in excised tomato leaflets only when supplied at the site of infection and caused some necrosis of veins and midribs. Treated leaflets took up more water than controls and uptake continued for 4 days.Water uptake by kinetin‐treated petunia leaf strips was less than by controls but continued for 6 days with only slight discoloration of the veins.It is suggested that the inhibitory effect of kinetin may be associated with a capacity to cause increased activity of ribonuclease in the tiss