Soil microbial biomass carbon (Cmic) and respiration were measured in soils from two long-term field experiments with recent sewage sludge applications. A moderately contaminated sludge was applied as received from the sewage treatment plant and after additional metal contamination. Generally, the low metal sludge had beneficial effects on Cmic and on the soil microbial activity. Higher heavy metal contamination of soils resulted in a substantial decrease in Cmic. The ratio even decreased when low metal sludge was applied. Soil respiration and especially the respiration per unit biomass (qCO2) increased with increasing amounts of heavy metals. Concomitantly the contribution of fungi to soil respiration increased. Judging from these results the qCO2 appears to be a sensitive indicator of effects that are resulting from the effects of heavy metals on the soil microflora.