Dynamic light scattering, steady-state fluorescence, NMR diffusometry and cryo-TEM have been used to gain more insight into the aggregation behaviour of LPS from Escherichia coli O55:B5. Knowledge of this behaviour of the amphiphilic LPS molecule is in many cases of importance for the design of experiments and interpretation of results when using LPS in solution. The aim of this work was to study the aggregation and determine the aggregate size of E. coli O55:B5. The mean hydrodynamic radius of the LPS aggregates was determined by NMR diffusometry and dynamic light scattering to 14 and 26 nm, respectively. The cryo-TEM technique revealed predominantly spherical aggregates of 9-19 nm. We wish to report 10 microg/ml as the aggregation start for LPS E. coli O55:B5 in PBS buffer, pH 7.2. We suggest that the aggregation is a continuous process that starts at 10 microg/ml and proceeds up to 300 microg/ml.