The experiments on accumulation of 99Tc by Elodea biomass showed that 99Tc activity concentration can reach 120 ± 6 Bq/g, with the concentration factor for 99Tc 2700 ± 500 L/kg. In experiments on 99Tc release, over 504 h about 82% of the total 99Tc activity was released into the water from plant biomass; most of 99Tc was released within the first 192 h. Results of chemical fractionation of the biomass show that 99Tc contained in biomass was mainly concentrated in the exchangeable and the adsorbed fractions (83%). Thus, the data obtained using chemical fractionation of biomass confirmed the experimental data on 99Tc release, which suggested that most of the biomass-bound 99Tc was adsorbed on the biomass surface. 99Tc tightly bound to Elodea biomass (fractions of organics and mineral residue) constituted just 17% of the total 99Tc activity. Decreased illumination of Elodea shoots during the experiment did not cause any reduction in 99Tc activity concentration or concentration factor. Results of chemical fractionation of the biomass grown under lower illumination conditions show that the percentage of 99Tc tightly bound to Elodea biomass (fractions of organics and mineral residue) decreased while the 99Tc of the adsorbed fractions decreased. Our results and data reported by other authors suggest that some part of 99Tc activity can be bioavailable to living organisms and that the portion of bioavailable 99Tc can be determined by a number of factors.