To obtain general information on the quantities of mineral nutrients in the microbial biomass of soils, we determined the carbon content of the microbial cells of 29 soils. Carbon data were converted to nutrient values using C to N, C to P, C to K, and C to Ca ratios established by measuring (1) the average C to mineral content of pure cultures of 24 species of soil microorganisms and (2) the relative contributions of bacterial and fungal cell populations to the microbial biomass of 17 soils. The biomass in 26 agricultural soils contained between 0.27 and 4.8 percent of the total soil C ( ≅ 2.5 percent) and between 0.50 and 15.3 percent of the total soil N ( ≅ 5.0 percent). The average quantities of N, P, K, and Ca in the microflora of the soils (upper 12.5 cm) were about 108, 83, 70, and 11 kg ha−1, respectively. In the L + F + H layers of three forest soils, between 0.52 and 0.91 percent of the total C and 0.16 and 0.22 percent of the total N were in the microbial biomass.