AbstractThe permeability of bacteriophage T4 and the change in T4 permeability caused by mutation to osmotic shock resistance are investigated here by quantification of the kinetics with which both a DNA‐specific probe (ethidium) and a protein‐specific probe [1,1′‐bi(4‐anilino) naphthalene‐5,5′‐di‐sulfonic acid, or bis‐ANS] bind to T4. In the case of an osmotic shock‐resistant mutant, T40s41, both ethidium and bis‐ANS bind with first order kinetics. The first‐order rate constant (k*) for both bis‐ANS and ethidium is a function of anion type and concentration. Adenosine triphosphate, phosphate, bisulfite, sulfate, and acetate anions all reducek* below thek* observed when chloride is the only anion. When chloride is the only anion at 25°C,k* values for binding to T40s41 are orders of magnitude abovek* values for binding to wild‐type T4 (T4wt). At 25°C,k* forT4wt is too small to measure, butk* for T4wt increases at 50–55°C to values approaching those measured for T40s41, without inactivating T4wt, when chloride is the only anion; during heating, T4wt is stabilized by both ethidium and bis‐ANS. Binding to T4wt is reversible at 50‐55°C, but not at 25°C. Equilibrium binding of bis‐ANS to T40s41 reveals 112 ± 24 sites per T4 capsid. Equilibrium binding of ethidium to T40s41 reveals both high‐ and low‐affinity sites previously observed in the packaged DNA of other bacteriophages. The ATP‐induced decrease ink* is not accompanied by a decrease in equilibrium binding. The following hypotheses are presented to explain the above data: (a) All detected bis‐ANS binding sites on T4 are interior to the outer surface of T4. (b) The value ofk* for both bis‐ANS and ethidium is controlled at the port(s) of passage through the outer shell of the T4 capsid. (c) The anions present controlk* values at the port(s) of entry, probably by controlling the size of this port. The effects onk* of phosphate explain the otherwise paradoxical observation [P. J. McCall and V. A. Bloomfield (1976)Biopolymers15, 2323–2336] that in a phosphate bu