Abstract: In previous articles^(1,2) the authors considered that the fundamental structure of the new type acid dye was the diaminotriphenyl-methane-sulfonic acid. Accordingly, the authors found another dye of this new type, i. e. patent blue V(PBV). Chloroform extract of the complex of PBV-amine shows greenish blue color, λ_max 632 nm. The preferable concentration of PBV used for determination of amine is 4×10^-3mol/L. The limits of detection of this dye for benethtropine(Ⅰ) and benactyzine(Ⅱ) are 16 ng/ml and 37 ng/ml respectively. The absorption coefficients (E_1cm^1% of these dye-amine complexes 37.6×10²(Ⅰ) and 22.0×10²(Ⅱ) are4.1～6 times as high as those of bromothymol blue (BTB), but for more hydrophilic amines, E_1cm^1% are lower than those obtained from BTB. The calibration curves are straight lines in the range of 0～6.5 μg/ml for Ⅰ and 0～7.5μg/ml for Ⅱ. With PBV concentration of 4×10^-3mol/L, the mole ratio of dye-amine complex in the chloroform extract for Ⅱ is PBV: Ⅱ=2:1 and 1:1 with 4×10^-4mol/L. In the case of Ⅰ, the mole ratio with different dye concentration (4×10^-4～4×10^-3mol/L) keeps constant at 3:1 (dye: amine). The coefficients of variation for determination of 0.2 μg/ml of Ⅰ and Ⅱ are within ±5%, while ±1～2% for 2.5 μg/ml, 5.0μg/ml and 10.0 μg/ml. PBV has the same properties and sensitivity as xylene cyanol (XC) and erioglaucine (EG). On the basis of the studies of three dyes, namely XC⁽¹⁾, EG⁽²⁾ and PBV, it is further considered that the following moiety is the fundamental structure of this new type acid dye: