A systematic approach to the characterization of the physicochemical properties of bacterial cell surfaces (isoelectric point, surface net and local hydrophobicity) has been presented using the same aqueous two-phase partitioning method that was applied for the characterization of protein surfaces (Kuboi et al., Solv. Extr. Res. Dev, Japan, 1, 42-52, 1995). It was found that the isoelectric point (pI) could be determined by cross partition method and that the pI of Escherichia coli cells is generally low. At the pI, the surface hydrophobicity (HFS) of the bacterial cells was determined by using the slope of the relationship between the hydrophobic differences of the two phases and the partition coefficient of the bacterial cells. The partition behavior of cells was found to be influenced by the addition of relatively low concentration of salts. The addition of nonionic surfactants of the Triton X-series led to an increase in the partitioning of the cells (Delta ln K-Cell) by causing a hydrophobic interaction between Triton molecules and the cell surface. This effect is probably due to the interaction between the hydrophobic groups of the surfactant and the local hydrophobic binding sites of the cell surface, such as the membrane proteins and phospholipid membranes. By using these parameters, such as the pI, HFS, and bin K-Cell the surface properties of bacterial cells which were exposed by heat treatment mere analyzed.