The effect of forest fertilization on soil microbial activity and biomass was studied in 11 coniferous forests with different site fertility. Soil microbial activity was measured as respiration rate and the microbial biomass was estimated using ATP content and substrate-induced respiration rate (SIR). At all sites, except those with the highest site fertility, the microbial activity and biomass were reduced in the fertilized treatments compared with the control. The general trend indicated that the lower the site fertility, the more pronounced was the reduction of microbial biomass and activity. Some possible explanations for this effect are discussed.