A method is described in which tissue excised from cotyledons of 10-day-old pumpkin (Cucurbita pepo L. ‘Alabama Gold’) seedlings is floated in the light on a phosphate-based medium suspected of containing a photosynthesis inhibitor. The discs sink in a few hours if photosynthesis is inhibited; otherwise they continue to float. The method is capable of detecting a 0.002 to 0.02 ppm concentration of the herbicide prometryne [2,4-bis(isopropylamino)-6-(methylthio)-s-triazine]. The method could be used both for screening chemicals for potential herbicide use and for monitoring water for pollution by photosynthesis inhibitors.