A photometric method based on measurement of optical density (A) at the absorption maximum of the flavonoid complex with AlCl3 in the region 400-430 nm are widely used for determination of the total content of flavonoids (FL) in plants. The amount of FL is expressed in units of rutin. However, in various literature sources conditions of extraction and complex formation with AlCl3 differ significantly. Our studies were carried out on the example of a very well-known and widely used medicinal plant Hypericum perforatum or the herb St. John's wort. The best extractant for FL extraction was 70% ethanol. But in this case nonpolar compounds, chlorophylls, hyperforins, anthracene derivatives were extracted from the herb St. John's wort simultaneously with FL which may also interact with AlCl3 and distort results of the analysis. Special noise could make chlorophylls, which intense Soret bands were observed in the spectrum in the region of analytical bands of FL (300 - 450 nm). Quercetin (aglycone) and rutin (glycoside of quercetin) were used as model flavonoid compounds. Solubility of FL was evaluated by the values of A in the maxima for quercetin (374 - 380 nm) and rutin (363 nm) of saturated solutions in various solvents. On the basis of the obtained data it was shown a scheme of a consecutive fractional extraction of substances that prevent the determination of FL (see on the scheme, LPh - liquid phase, SPh - solid phase). The spectra in the region of 200 - 700 nm of extracts from petals of flowers and leaves of the herb St. John's Wort obtained by consecutive extraction with hexane, chloroform, and an aqueous solution of 0.1 M AlCl3, ethanol and ethanol total extract were studied. Spectra were recorded on a Lambda 25 spectrophotometer (PerkinElmer, USA, 2005). The resulting spectral data showed that the proposed scheme for determination of the total content of flavonoids in the aqueous extract 0.1 M AlCl3 simplifies the analysis and specified the results. Chlorophylls and anthracene derivatives were extracted from Hypericum perforatum by ethanol along with the FL, and interacted with AlCl3, which led to changes in the spectrum of the initial extracts. Thus, we proposed the scheme of a consecutive fractional extraction of the FL from plant material for determination of the FL total content and it is recommended for analytical practice.scheme