Critical heavy metal levels for microorganisms in mineral agricultural soils are derived from laboratory and field studies reported in the literature. Critical levels are defined as the heavy metal concentrations or the heavy metal loads which have been shown to negatively affect microbial populations, or microbially mediated processes. No attempt is made to assess the impact of the observed negative effects on functioning of the soil ecosystem, or on soil fertility and productivity. Most emphasis is placed on effects observed in long term field experiments. The heavy metals arsenic, cadmium, chromium, copper, mercury, nickel, lead, vanadium and zinc were considered, but there was only sufficient evidence to determine critical levels for copper, zinc and nickel with some degree of confidence. There was a lack of clear evidence from field studies for the other heavy metals. The critical levels for copper, nickel and zinc derived from the laboratory studies were similar to those derived form field studies, when expressed as relative increases over soil background concentrations. The critical levels for copper, nickel and zinc were converted to maximum allowable concentrations (MAC). The MAC values are intended to aid the safeguarding of the functioning of the soil ecosystem, and the conservation of long term soil fertility and productivity against negative effects of long term metal accumulation on microorganisms in agricultural soils. the proposed MAC values for copper, nickel and zinc for Swedish agricultural soils, based solely on effects on soil microorganisms, are 30, 50, and 80 (mu)g g(sup -1) soil, respectively. (89 refs., 5 tabs.) (au).