This paper reviews the effects of heavy metals on microbial biomass in metal-polluted soils. Laboratory and field investigations where metals were applied ass inorganic or organic salts demonstrated a significant decline in the size of s oil microbial biomass. In most of the cases, negative effects were evident at metal concentrations below the European Community's (EC) current permissible metal levels in the soil. Application of metal-enriched sludges and composts caused significant inhibition of microbial biomass at surprisingly modest concentrations of metals in the soil that were indeed smaller than those likely to decrease the growth of sensitive crop species. On the whole, relative toxicity of metals decreased in the order of Cd>Cu>Zn>Pb, but a few exceptions to this trend also existed. A significant decline in the biomass carbon to organic carbon ratio(Cmin/Corg) in metal-polluted soils indicated that this parameter can serve as a good indicator of the toxicity of metals on soil microflora. The knowledge regarding the response of soil biota to metal interactions and the factors affecting metal toxicity to soil microorganisms is still very limited and warrants further study.