The Space Transportation System uses a triiodide quaternary ammonium strong base resin to prevent microbial contamination of the crew's drinking water. Current plans for Space Station Freedom use the STS resin for microbial control in drinking water. Another use for this water is in the “salad machine” to grow vegetable plants hydroponically. Our experiments demonstrate that leaf lettuce (Lactuca sativa) grown in nutrient solution treated with the triiodide resin and it's next higher homologue, pentaiodide, result in greatly reduced growth or death. The triiodide and pentaiodide treatments reduced plant fresh weights to 0.2% and 0.04% of the controls respectively. Tissue analysis by neutron activation showed an iodine concentration of 0.47% to 0.6% in the experimental plants. Nutrient solution analysis showed an average residual concentration of 38 and 65 mg/l iodine at the end of the 30 day experiments for triiodide and pentaiodide treatments respectively. Consequently, these resins should not be used alone in conjunction with plant growth systems.