Dehydrogenase activity (DHA) of the microbial biomass was measured in sewage sludge amended soil samples collected from the Braunschweig experimental site, Germany. The site had received additions of sludge with or without heavy metals at two application rates (100 m3/ha per year and 300 m3/ha per year) on soils of ‘low’ (4.8–5.8) and ‘high’ (5.4–7.0) pH since 1980. DHA was found to be a sensitive and precise assay for determining the effect of heavy metals on substrate-induced (glucose) microbial biomass in sewage sludge amended soils. Effects on DHA were determined in relation to heavy metal concentrations and other soil factors. Addition of relatively uncontaminated sludge enhanced DHA, but this was dependent on the level and type of sludge addition. Adverse metal effects were only significant in the most contaminated soils where sludge had been added to the ‘high’ andd ‘low’ pH treatments at Braunschweig. However, these effects were small compared to the effects of high rates of sludge addition alone, despite exceeding statutory limits for Zn and Cu, where concentrations reached 341 and 99 μg/g, respectively.