The efficiency of phytoremediation might be highly affected by plant-associated microorganisms, and understanding of the underlying mechanisms is still a great challenge. The primary aim of this study was to evaluate the efficiency parameters for Cd2+ accumulation in the biomass of willow (Salix viminalis) as well as to define the biochemical response of the host plant when it is inoculated with selected bacterial strains (Massilia sp. and Pseudomonas sp.) or saprophytic fungus (Clitocybe sp.) under controlled in vitro conditions. Inoculation of plants with bacterial strains affected the efficiency of phytoremediation process and was expressed as the quantity of accumulated Cd (Q), the bioaccumulation factor (BCF) and the translocation index (Ti); however, the effect was strain and plant organ specific. The level of hydrogen peroxide (H2O2), which is both an indicator of plant response to biological and/or abiotic environmental stress and a molecule involved in plant-microbial interactions, decreased under the influence of Cd2+ in uninoculated plants (plant growth was inhibited by Cd2+) and increased in the inoculated variants of plants growing in the presence of Cd2+ (microbiologically stimulated biomass). The saprophytic fungus Clitocybe sp. generally stimulated biomass and increased the level of H2O2 synthesis in all the investigated plant organs and variants of the experiment. We suggest that determination of phytoremediation efficiency, and biochemical response (H2O2) of the host plant under in vitro conditions can help in predicting the final effect of plant-microbial systems in further field trials.