Almost all previous reports on plant esterase (EST) isoenzyme analysis appeared to have accepted that the relative staining strength (RSS) of electrophoretical bands represented the relative activities and contents of the corresponding isoenzymes. Nevertheless, we found in studying that the RSS changed with different staining agents. Plant materials used in this experiment were O. sativa L. male sterile line 765- 5A, one accession of O. grandiglumis prod, and their interspecific hybrids, 230-1, 230- 2, and 230-3, obtained by the embryo-rescue. The vertical polyacrylamide gel thin layer electrophoresis technique and the staining procedure for EST isoenzymes were described by the Biochemical Teaching and Research Section of Beijing Agricultural University (1988). Fully expanded leaf blades of last 2nd leaf but one were sampled from the plants grown under natural conditions.