Abstract <p>The study characterizes the effect of polymer soil ameliorants on changes in toxicity of agro-sod-podzolic soil contaminated with a complex of heavy metals (HMs). Hydrolyzed polyacrylonitrile (HPAN) and a binary composite of HPAN + potassium humate (HPAN + PH, 1 : 10) were applied at a rate of 0.2% (on a dry matter basis) to soils with model contamination with salts of copper, zinc, lead, and cadmium at levels 2, 4, and 6 of tentative permissible concentrations (TPCs). The effect of polymer formulations on reduction of heavy metals toxicity manifested itself to varying degrees depending on soil contamination level. The bacterial standard bioluminescent bioassay and phytotesting on the seedlings of higher plants <i>Sinapis alba</i> L. showed a positive effect of HPAN + PH composite at the highest contamination level, 6 HM TPC. Bioindication of the functional state of soil microbiota showed that ameliorants lowered the microbial metabolic quotient (to the permissible level of 2 µg C–CO₂ mg⁻¹ C_mic h⁻¹), which is an important evidence of their positive effect on the soil microbial community. Laboratory biotesting of uncontaminated soil, as well as at relatively low levels of HM contamination, revealed the toxicity of the aqueous extract of samples with ameliorants (root growth delay and decrease in bioluminescence in test-bacteria). The need is emphasized for additional analysis before recommending the use of the ameliorants for detoxification of contaminated soils considering the results obtained in the study.</p>