Natural aggregates 01' topsoil samples of six British soils (Handope, Ragdale, Evesham, Denchworth, Flint and Salop) were incubated with added nutrients and changes in their stability to wet sieving determined. Under anaerobic incubation the type of nutrient added had little effect on stability. Aerobic incubation with glucose gave a larger proportion of stable aggregates than with water. With peptone stability usually decreased, and occasionally increased, which may indicate differences in the binding mechanisms of the soil particles. Both sterile and unsterile aggregates incubated with water became more stable than controls; the rapidity of this change suggested a physical. rather than microbiological cause. Very stable grassland aggregates changed little on aerobic incubation with water, but b e c a m more stable with solutions of glucose or peptone. Artificial aggregates made from the Same soil developed considerable stability to water on aerobic incubation. but remained almost totally unstable when incubated anaerobically.