Propel, a product containing a specific isomer of lactic acid that exercises PGR activity in cotton, was evaluated at the 16-oz application rate using five cotton growth stages. In a high yield environment Propel produced substantial yield increases ranging from 33.6 to 121.2 pounds of lint per acre. Only the 45 DAE treatment (10.3% increase) was significant at the 5% level. Other South Texas locations produced no significant improvements in lint yields or fiber quality, but trends for increase were usually observed at 45 and 60 DAE. Benefits from Propel were attributed (89%) to increased boll weight, which was theorized to be due to repartitioning of carbohydrates. Exact mode of action is unknown. Turnout was higher for Propel derived bolls and more green bolls were observed at harvest when yield increases occurred. Overall increases at 45 DAE were 3-10%. High variability among replicates was observed at some locations in 1991.