The "Sennosides Reference Standard (Control 951)" was prepared, which is intended to be used for the fluorophotometric assay of sennosides content in the preparation of "Sennosides". In this assay hydroxylated mono- and dianthraquinone glucosides are chelated with boric acid, and the fluorescence intensity of the chelate is determined against that of the Reference Standard (RS). In the establishment of this RS, sennosides content in the candidate material must be determined accurately by fluorophotometry. The Sennoside AB for assay, prepared as an equimolar mixture of the purified sennoside A and Sennoside B, was used as the RS for the fluorophotometry. Based on the above concept, sennosides content in the candidate was determined as calcium salts to be 60.1 +/- 1.6% by the fluorophotometry. Thus the sennosides content of this Sennosides RS was certified to be 60%. Separately, contents of Sennoside A (SA) and Sennoside B (SB) in this candidate were determined by using HPLC. As a result, the sum of SA and SB was estimated to be 38% as free acids. Thus it was suggested that about 20% of dianthraquinone glucosides other than SA and SB and anthraquinone glucosides may be included in this Sennoside RS as free acids. Analytical results on the USP Sennosides RS were also shown and discussed, compared with the present Sennosides RS.