This paper reports results of studies on the possibility of microbiological utilization of rubber waste products with the use of Thiobacillus ferrooxidans bacteria. Both raw and chemically-modified rubber vulcanizate were found to be toxic towards bacteria. The toxicity of particular components of the rubber was determined and the most toxic agent was found to be the anti-ageing Dusantox 6PPD (N-(1,3-dimethyl-butyl)-N-phenyl-p-phenylenediamine).