The rate of ammonification of arginine-N, after addition to soil is proposed as a rapid index of soil microbial activity. Correlation between arginine ammonification, biomass C, heat output, soil ATP content and soil protease activity, were assessed on twenty-two agricultural and grassland soils collected from thirteen habitats in Germany and Austria. There were highly significant, positive correlations between all methods (r > 0.9). Close relationships were also found between these methods and total soil organic matter content (r = 0.862-0.957), but not with soil pH or the viable microbial count. Our results show a close relationship between the microbial biomass and N-mineralization in soils, and suggest arginine ammonification as a valid indicator of microbial activity in soil.