A new bioassay is described which is based upon the elongation response of the second leaf sheath of rice plants to gibberellins. Rice seedlings, at their first leaf stage, were treated by applying gibberellins to coleoptils in a 1μl drop of 50% acetone using a micropipette. Agar blocks can be used in place of test solution. By means of this method, five cultivars of rice, including one normal and four dwarfs of different simple recessives, were tested for their response to gibberellins A1 to A5. Gibberellins A1 and A3 are of high potency in these cultivars. The dwarfs differ in the specificity of their response to gibberellins. For example, 'Tan-ginbozu' responses well but 'Waito-C' much less to Gibberellin A5. These dwarfs can be used as a 'mutiple plant assay' to detect gibberellins with similar structures. Gibberellin A3can be bioassayed in the range from 0.05 up to 100mμg using the 'Tan-ginbozu' dwarf. The rice test using microdrop application was less susceptible to inhibiting substances such as Abscisin II, fusarinic acid, and indoleacetic acid than the test using the application in growing media. Because of this and because of the rapidity and high sensitivity of the test, it has been successfully applied to the estimation of gibberellin-like substances in extracts from small quantities of rice shoots and Pharbitis nilseeds. The chromatographic behavior of the active zone of rice shoot material was similar to that of gibberellin A3. Bioassays on extracts ofPharbitis nilseeds using the 'Tan-ginbozu' and 'Waito-C' dwarfs indicated the presence of gibberellin-like substances with differential effects on the two dwarfs.