The influence of heavy metal contamination on the efficiency of conversion of fresh substrates into new microbial biomass in a pasture soil was examined. Three soils covering a range of chromium, copper and arsenic concentrations, and an uncontaminated control soil, were amended with [U-C-14]glucose and incubated for 28 days. During incubation, microbial biomass C-14 was determined using the fumigation-extraction technique. The amounts of (CO2)-C-14 evolved during incubation were monitored, and residual C-14 concentrations were determined. Throughout the incubation, the microbial biomass-C-14 formed following addition of glucose was consistently lower in the metal-contaminated soils than in the uncontaminated control soil. Soils differed significantly in their rates of (CO2)-C-14 evolution. More glucose-derived (CO2)-C-14 was evolved from contaminated soil than from the uncontaminated control. The ratio of both (total respired C):(total biomass-C) and (respired (CO2)-C-14:(biomass-C-14) was greater in the contaminated soils than in the uncontaminated soil. The results suggest that the microbial biomass in soils contaminated with heavy metals are less efficient in the utilization of substrates for biomass synthesis and need to expend more energy for maintenance requirements.