Tomato and lettuce seedlings were exposed to 1 or 3 mm of simulated rainfall containing the auxin-type herbicide 2,4-D iso-octyl ester. They were then evaluated weekly for damage according to symptoms and chlorophyll content. In addition, dry matter production was determined at the end of the three- week growth period. The herbicide threshold concentration for causing the symptoms‘stem, leaf and petiole bending’on tomato seedlings and‘duck's foot-type leaves’on lettuce seedlings was found to be 10 pg L−1. Significant reduction of the dry mass of the above-ground plant parts began with 10 ng L−1of the herbicide. Further dry mass seems the only parameter measured that is sensitive to application volume. There was a significant decrease in chlorophyll content at 10 mg L−1of the herbicide. It was deduced that neither chlorophyll content nor dry mass can be used as markers for auxin-type herbicide damage.