Sensitivity of the micro-drop assay with dwarf rice cultivars (Oryza sativa L., cv., Tan-ginbozu and cv. Waito-C) to gibberellins (Gas) was increased approximately thirty-fold by the use of assay plants that had been treated with uniconazole (S-3307), i.e. an inhibitor of the biosynthesis of Gas. The spectrum of detectable Gas was not changed by that treatment. In addition, the treatment with S-3307 counteracted in part the inhibition of growth of both cultivars by abscisic acid. Thus, the modified micro-drop assay is recognized to be very useful for the detection of minute amounts of Gas in plant extracts.