Microbial biomass (C(mic)) as a labile component of soil organic matter with a short turnover time may be a sensitive indicator of changes in the content of soil organic carbon in soil. At present, the fumigation methods and the method of substrate induced respiration seem to be the most suitable. The objective of the present study was to show the variability of (C(mic) during vegetation season and to establish relation between C(mic) and C(org) in a set of agricultural soils from different climatic regions. The range of C(mic) fluctuation during vegetation season was expressed by the index maximum C(mic) /minimum C(mic), which was ranging from 1.5 to 16.6 in dependence on the season and agricultural practices (Tab. I). C(mic) established on these localities on early spring and late autumn sampling data were closely correlated to mean values of C(mic) calculated from monthly samplings (Tab. II). C(mic) was also established in soil samples of one type (cambisol) taken in early spring in one climatic region but differing by agricultural practices. C(mic) ranged from 122 to 560 mug C.g-1 in dependence on C(org) content while no relation with N(tot) was found (Tab. III). This could be connected with the fact that results were obtained in soils fertilized with nitrogen where C(mic) was not limited by nitrogen availability. C(mic)/C(org) ratio in these soils were fluctuating from 0.69 to 2.38 independent on the factors under study (Tab. III). C(mic) determined in soils with different texture (sandy loam, loam and clay loam) and from various climatic regions was related to C(org) (Tab. IV), though no dependence on the clay content and other investigated factors was found. A possible explanation is discussed.