The nature of the ultrafiltration resistance of Escherichia coli broth harvested at different culture times was investigated. E. coli broth was harvested at the point of glucose depletion (0-h aged broth) and at 12h after that (12h aged broth). The filtration resistance of whole broth nearly doubled during the 12h aging. In the case of the Oh aged broth, the dominant resistance was from the deposited cells. However, for the 12h aged broth, an interaction between cells and soluble broth components determined the filtration resistance. If either were missing, the resistance increase was markedly less. The cause of the interaction appeared to be a change in the cell (surface) during the aging, leading to binding with some broth component(s) that increased the resistance. The increased resistance occurred both when broth components were present during cake formation and when exposure came after cake deposition from washed cells. The interacting soluble component permeated a 0.16 mum membrane and was retained by a 300 kDa MWCO membrane. Since DNA would show such permeation and DNA added to aged, washed cells increased resistance in a similar fashion as occurred with aged broth, it is likely that DNA released from cells contributes to the interaction with aged cells.