Activated sludge from a municipal sewage treatment plant was cultivated in a bioreactor (vol. 10 L, COD 172 g/L, residence time 6.7 h) in presence of CdSO4 or Cd(OAc)(2) (0.1-8.1 or 10-100 mg/L, resp.) to study the toxicity of Cd ions on dehydrogenases of the sludge-contained microorganisms by using 2,3,5-triphenyltetrazolium chloride electron acceptor. A decrease in the activity of dehydrogenases was obsd. The toxicity of CdSO4 was higher than that of Cd(OAc)(2).