Current knowledge of heavy-metal effects on the biology of the mor horizon in Swedish forests is reviewed. Elements mainly considered are Cu, Zn, Cd, Pb, and Hg, though Cr, Ni and V are also discussed to some extent. Mor (rawhumus) was chosen because it is the most widespread type of organic matter (humus) horizon in Sweden. Critical concentration is defined as the lowest total content (microgram per gram dry weight of the mor) which has proved to exert or, for excellent reasons, is suspected to exert a negative influence on the biology of the mor. Alternatively, the critical concentration is defined as the degree of increase above a current base-line concentration in the mor, to which the soil biology is adapted, necessary to evoke adverse effects. The effect of each metal is mostly considered in isolation. It has usually not been possible, due to lack of knowledge, to take into consideration interactions between metals, or influence of other soils properties on the biological effects. Nor have shock effects on soil microorganisms caused by other components than metals, contained in various waste products applied to mor horizons, been considered. A summary of critical concentrations to different processes and organism groups is given. (15 refs., 2 figs., 7 tabs.) (au).