Effect of ABA, GA3, zeatin (Zea) and IAA on the inclination activity was examined by a rice lamina inclination assay using a Korean cultivar, Tongjin. Treatment with ABA, GA3, Zea or IAA alone failed to incrase the inclination response significantly at the concentration tested from 0.1 ppm to 10 ppm. However, treatment with 0.1 and 1 ppm ABA in the presence of brassinolide inhibited the inclination activity induced by treatment with brassinolide alone. And treatment with 0.1 and 1 ppm GA3, or Zea in the presence of brassinolide stronly inhibited the inclination activity induced by treatment with brassinolide alone. On the other hand, the inclination activity by treatment with brassinolide alone was clearly promoted by treatment with 0.1 and 1 ppm IAA in the presence of brassinolide. Based on the synergistic effect induced by treatment with BR and IAA, we could develope an improved rice lamina inclination ssay whose minimum detectable concentration of brassinolide is 0.00001 ppm/petri dish. The minimum detectable concentration in our assay was five times as low as that of the previous rice lamina inclination assay.