Thymidine and leucine incorporation into macromolecules of soil bacteria extracted by homogenization-centrifugation were measured after sizefractionation of the bacterial suspension through different sized filters (1.0, 0.8, 0.6, 0.4 p~m). The specific thymidine incorporation rate was highest for the unfiltered and 1.0 p~m filtered suspensions (approximately 10 x 10 -21 tool thymidine bacteria-1 h-1), but decreased to 1.39 × 10-21 mol bacteria-~ h-1 for bacteria passing the 0.4 p~m filter. The proportion of culturable bacteria (percent colony forming units/acridine orange direct counts) also decreased with bacterial cell size from 5.0% for the unfiltered bacterial suspension to 0.8% in the 0.4 p~m filtrate. A strong linear correlation (r 2 = 0.995) was found between the specific thymidine incorporation rate and the proportion of culturable bacteria. Leucine incorporation gave similar results to the thymidine incorporation. No effects of cell size on the degree of isotope dilution or unspecific labeling of other macromolecules were found either for the thymidine or the leucine incorporation technique. These data indicate that small bacteria, although more numerous than larger ones, not only constitute a smaller proportion of the soil bacterial biomass than larger bacteria, but also contribute to a lesser degree to carbon transformations in soil.