The effect of microwave irradiation on two soils of different carbon content was evaluated with the intent of investigating the potential of using this treatment to estimate soil biomass. After irradiation for various time intervals, the soil was incubated at 25 + or - 0.1 C for 10 days. The amount of CO2-C produced with the two different soils remained constant, and little or no difference was observed for microwave treatment ranging from 1-20 min (4.2 X 10(sup 4) -8.5 X 10(sup 5) J). When the CO2-C results of this study were used to calculate biomass-C estimates, the Sierra and Garden soils were 29.5 + or - 3.0 and 26.9 + or - 3.9 mg C 100/g dry soil, respectively. This compared with values of 35.8 + or - 0.7 and 28.8 + or - 1.2 mg C 100/g dry soil obtained by chloroform fumigation. These results indicate that, for the soils tested, a sufficient number of microorganisms survived microwave irradiation to metabolize nutrients released from killed cells during the controlled heating process. The approach appears to provide biomass estimates comparable to chloroform procedure without the use of fumigants. (Copyright (c) 1988 Kluwer Academic Publishers.)