Messenger™ is the first of a new class of crop protectants which contain the active ingredient, harpin. Harpin, an extracellular protein isolated from bacterial plant pathogens, activates a plant’s natural defense mechanisms by inducing systemic acquired resistance (SAR), thus providing resistance to a broad range of diseases and pests. Preliminary trials have shown that cotton (Gossypium hirsutum L.)yield might be improved with foliar applications of Messenger™ . In 2000, two field studies were conducted in eastern Arkansas to determine the effects of Messenger™ on cotton yields. In field study 1, treatments consisted of an untreated control, seed treatment (2 oz/100 lb seed), foliar treatment (2.23 oz/acre),and seed + foliar treatment. In study two, treatments consisted of an untreated control and seven foliar treatments at a rate of 2.23 oz/acre applied at various timing intervals. No significant differences in yield were encountered between treatments at either location. Widespread potassium deficiencies were observed at both locations. It is possible that the excessively hot, dry conditions along with the nutrient deficiencies, masked potential yield increases in the field. A growth chamber study was conducted to investigate the effects of Messenger™ under controlled conditions. Treatments consisted of two rates: 2.23 and 4.46 oz/acre, and two timings of Messenger™ application: beginning at 2 ^nd true-leaf and pinhead square. While no significant differences were observed in physiological parameters, treated plants had significantly more nodes and squares at first-flower, suggesting that Messenger™ enhanced cotton growth and yield potential.