The fluorescent probe, 8-anilino-1-napthalenesulfonate (ANS) binds to Escherichia coli, showing an enhanced fluorescence. The interaction of colicin E1 with sensitive cells causes an increase of about 100% in the fluorescence of the bound ANS, and this change at equilibrium has an apparent “all-or-none” nature as a function of E1 multiplicity. Approximately 6 to 8% of the ANS is bound to the cells at equilibrium. The colicin E1-induced fluorescence increase can be attributed partly to an increase in ANS binding and partly to an increase in the fluorescence yield of the bound ANS. The kinetics of the E1-induced fluorescence increase in sensitive cells are very similar to those of the adenosine triphosphate decrease. The phosphorylation uncoupler p-trifluoromethoxy-carbonylcyanidephenylhydrazone also causes a large change in the fluorescence of bound ANS. Colicin E2 or E3 does not cause any fluorescence change, nor does colicin E1 cause fluorescence change with a colicinogenic strain. ANS appears to be a probe of structural or conformational change in the cell envelope that is closely associated with the colicin E1-induced adenosine triphosphate decrease.