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The elemental composition of cells is an important physiological characteristic of an organism. Of interest are the role of different elements, their interactions, and the distribution of elements among the cell fractions of cyanobacteria. The cyanobacteria under study are widely used in phytobiotechnology. Biomass enriched with bioavailable elements can be obtained because of the plasticity of the metabolism.It was shown in [1-3] that vital elements (Mo, Se, and Zn) can enter the cells of cyanobacteria Spirulina platensis and Nostoc commune. We used a set of analytical methods (ICP-AES, PAAS, and ETAAS) to determine the elemental composition of cyanobacteria upon the enrichment of biomass with these elements and to study element distribution among the cell fractions.The procedures developed for the determination of macro-and microelements in biological samples [4,5] were adapted for the analysis of cyanobacterial biomass. Special attention was given to the development of an ETAAS procedure for the determination of Mo a Se in cell fractions containing chloroform and methanol. Selenium in fractions was determined by fluorimetry after extraction with 2,3-diaminonaphthalene [6-7]. Before the extraction, biomass and the separated fractions were ashed by wet digestion in a mixture of HNO 3 and HClO 4 . It should be noted that this procedure is complex and very laborious. EXPERIMENTAL Reagents and solutions. GSO-BV-1 certified reference solutions of individual elements (Bogatskii Physicochemical Institute, Ukraine) were used for the determination of elements by ICP-AES. A stock solution of selenium (1 mg/mL) was prepared by dissolving 1 g of metallic selenium in HNO 3 followed by evaporation to remove nitrogen oxides and dilution with twice-distilled water to a volume of 1 L. Working solutions containing from 0.005 to 0.1 mg/L selenium were prepared by the successive dilution of the stock solution with 20% HNO 3 . A stock solution of molybdenum (1 mg/mL) was prepared by dissolving ammonium molybdate in twice-distilled water. Working solutions contained 0.01-0.1 mg/L Mo in 20% HNO 3 of highpurity grade.Apparatus and working conditions. Atomic-emission determination was performed on a Thermo Jarrell Ash ICAP-9000 ICP-AE spectrometer with a 48-channel polychromator and a diffraction grating 10-15 lines/mm its inverse linear dispersion was 0.92 nm/mm. The polychromator is characterized by a spectral range of 190-800 nm; high-frequency generator capacity of 2.0 kW, and working frequency of 27.12 MHz. The flow rates of the cooling, plasmaforming, and carrier gas (argon) were 15.0, 0.6, and 0.4 L/min, respectively; solution flow rate was 2 mL/min; the height of the plasma observation zone was 17 mm, and exposure time was 10 s. Scanning the spectra of elements near their analytical lines revealed spectral interferences in the solutions of cyanobacterial biom-ARTICLES Abstract -An approach to studying the elemental composition of cyanobacteria Spirulina platensis and Nostoc commune using a set of complementary analytical method...