The activity of uncultured acidobacteria was monitored in Dutch grassland soils by quantifying their ribosomes. These bacteria were detectable by five different 16S rRNA RT-PCR products in temperature gradient gel electrophoresis fingerprints. The ribosomes in surface soil samples were quantified with multiple competitive RT-PCR along a 1.5-km transect through the grassland. In total, the five members of the acidobacteria were estimated to contribute 4 x 1010 to 1 x 1011 ribosomes g soil-1, representing 7-14% of all bacterial ribosomes. These results indicate that ribosomes from acidobacteria are continuously present and abundant in soil and might contribute significantly to microbial activity in soil.