Fungal and bacterial substrate-induced respiratory activities have been distinguished in gray forest and chestnut soils in various ecosystems (forest, grassland, tillage, unused land, and shelterbelt) using the antibiotics cycloheximide and streptomycin. The optimal inhibitory concentrations of the antibiotics, added separately and in combination; the preincubation time of the antibiotics with the soil before glucose addition; and the mass of added inert material (talc) have been determined. Based on the results obtained, the inhibitor additivity ratio (IAR) has been calculated for the antibiotics. With the IAR differing from 1.0 by a value of more than 5%, the fungal and bacterial substrate-induced respiratory activities can not be distinguished reliably. Respiration measurements show that the microbial communities of natural ecosystems are dominated by fungi (81-95% on average). The smallest amount of fungi (54-59%) is found in the tillage ecosystem.