Abstract: The aqueous solution of Atabrine(hydrochloride salt or methanesulfonate)reacts with potassium cadmium iodide solution to form a crystalline precipitate with the composition C₂₃H₃₂ON₃Cl.CdI₄: _χH₂O.The sensitivity is 1:500,000.Based on this fact,a procedure for the assay of Atabrine and its preparations is proposed as follows: Weigh accurately 0.8 miliequivalent of atabrie into a 50 ml volumetric flask,and dissolve in 30 ml distilled water.Add 10 ml of M/10 potassium cadmium iodide solution with continuous shaking and dilute with distilled water to the mark.Mix well and then let stand for 30 minutes with occasional shaking.Filter through a dry filter paper and reject the first 10 ml.filtratc. Pipet 25 ml of the subsequent filtrate into a 125 ml.Erlenmeyer flask,the excess cadmium ion is titrated with M/50 2 Na-EDTA solution by a similar method as described in the authors'previous work. The result obtained by the above method is in agreement with those obtained by the gravimc- tric method. The interference of magnesium stearate in tablets may be eliminated by the masked and dema- skcd process,therefore,the method can be applied for the assay of atabrine tablets.