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Indications of possible negative effects of lead (Pb) and mercury (Hg) on microbial respiration in Southern Swedish forest humus layers led to experiments on dose-response relationships by additions of metal salts in the laboratory. Respiration rates and weight loss due to decomposition of organic material were measured. For relevance to field situations metal doses were low, the time span was long, 550 days including freeze storage, and microbial activity was kept up by plant litter additions. We looked for effects of Pb and Hg at levels moderately elevated above the Southern Swedish reference, as well as combined effects of Pb + Hg. A reduction in respiration and decomposition of 10% was found at about 225–245 μg g−1 of total Pb, i.e. at a Pb level elevated 3.5 times. Although small effects of Hg were found even at the lowest dose level, 10% inhibition of microbial activity appeared temporarily at about 2–3 μg g−1 of total Hg, i. e. at 6–8 times the reference level. There were no long-term additive effects of Pb and Hg on decomposition. Type of anion had a strong influence on the test, chlorides of Pb being more toxic than nitrates. Long-term monitoring and maintenance of microbial activity during the experiment were prerequisites for the occurrence of effects at low metal levels.