[en] The aim of this study was to investigate the change of Pu oxidation states due to interaction with aerobic bacteria and fungi at low pH under laboratory conditions. Microorganisms were isolated from samples collected from the low-level radioactive waste repository within the confines of Ignalina NPP. Abilities of the fungi (Absidia spinosa var spinosa Lendn. and Paecilomyces lilacinus Thom Samson) as well as Gram-positive bacteria (Bacillus mycoides and Micrococcus luteus) and Gram-negative bacterium I-m1 to transform the oxidation states of Pu under aerobic conditions were investigated. Oxidized and reduced Pu was tested using two radiochemical procedures. The amount of reduced and oxidized Pu was determined by measuring alpha activity after radiochemical separation. The results have shown that all bacteria and fungi can very slightly alter oxidation states of Pu due to their microbial activity. All the microorganisms tested demonstrate quite a fast process of Pu biosorption under the experimental conditions. (author)