Based on the development of a direct DNA extraction method from soil, an estimation of soil microbial biomass by measurement of extracted soil DNA is presented. Chloroform-labile carbon (CLC) and nitrogen (CLN) determined by chloroform fumigation methods have been the major parameters of soil microbial biomass, but to date, there have been few studies of the relationship between DNA and CLN among soils with different characteristics. We applied DNA assays to the estimation of soil microbial biomass in various types of soil in Japan. The quantitative relationships among DNA, CLC and CLN released by the chloroform fumigation-extraction method were examined in 35 soil samples including five different soil types. The amount of DNA was moderately correlated with CLC (r = 0.652, n = 35), but there was no significant correlation in one of the soil types (fluvisol) representing a typical soil in Japan (r = 0.124; n = 15). Moreover, there was a strong correlation between DNA and CLN (r = 0.887; n = 35), and the correlation was high in every major soil type. These findings raise the possibility that DNA could be used to estimate biomass N even among different kinds of soil, with a conversion factor of 2.75 for DNA (mu g g(-1) soil).