The breaking force of single polystyrene latex aggregates has been measured by micromanipulation. Aggregates were made by mixing primary particles at their isoelectric point using a vortex mixer or Brownian motion. The aggregates made by the former method had a mean breaking force of 51 +/- 3 mu N, whilst those by the latter had a mean breaking force of 5.3 +/-0.5 mu N for samples taken at aggregation times of 20 min to 6 h. This clearly demonstrates that the micromanipulation technique is suitable for characterising the breaking force of particle aggregates made under different aggregation conditions, and that the way collisions of primary particles were created had a significant impact on the breaking force of the aggregates.