Fumigation-extraction was evaluated for the measurement of microbial biomass-S in soil. Following fumigation in CHCl3, S was extracted using 10 mM CaCl2 at a soil-to-extractant ratio of 1-to-2 (w/v). These conditions were shown to be favourable in a range of soils containing extractable-S below 30 mu g S g(-1) soil. Extractable-S was measured by ion chromatography following H2O2 digestion. The increase in extractable-S following fumigation (the flush) was converted to biomass-S using a conversion factor (k(5)) determined using S-35-labelled biomass-S. Using a soil-to-extractant ratio of 1-to-2 gave a value for k(5) of 0.31.This methodology was shown to be reliable for soils subject to arable, grassland or woodland management, with pH values above 3.7 and clay contents of 4-60%. The amounts of biomass-S measured in these soils were in the range 2.9-20 mu g S g(-1) soil. The data also showed that the amounts of biomass-S in soil are related to biomass-C and dependant upon soil management and fertilisation.