The action of some polyhexamethylene biguanides upon Escherichia coli has been investigated. An amine-ended-dimer (n = 2), a polydisperse mixture (n = 5.5) and a high molecular weight fraction (n greater than 10) of the compounds were employed. The three compounds caused damage and disruption of the cell envelope indicated by changes in permeability towards the dye 2-p-toluidinylnaphthylene-6-sulphonate. This damage was shown to be reversible below certain critical concentrations of the biocides. Prolongation of treatment time did not increase the extent of non-recoverable injury nor influence those critical concentrations of biocide initiating it. Recovery processes were rapid and complete within 1 min. The inclusion of an inhibitor of respiration, antimycin A, in the recovery medium did not reduce the rate of recovery by damaged cells.