If a suboptimal concentration of the necessary ions is employed when T2 virus attaches to host cells, the resulting binding is largely reversible, even at 37°C.

Reversible T2 attachment to host cells leaves the cell undamaged and capable of normal reproduction. Irreversible attachment results in death of the cell.

Zn⁺⁺ exercises a specific inhibitory action on the invasion of E. coli B by T1 virus. The virus can still attach to the host cell at a rate closely approximating the maximum value, but the reaction remains reversible and the cell is protected against permanent damage.

The protective action of the Zn against T1 invasion is exerted through an action on the cell, rather than on the virus.

Studies of the uptake of radioactive Zn⁶⁵ show that cells become completely immune to T1 invasion when, on the average, 4 x 10⁷ atoms of Zn have been taken up by each cell.

Cells killed by ultraviolet irradiation still bind T1 at the maximum rate, but the reaction is reversible even when taking place at 37°C., in optimum salt concentration.

The tryptophane-deficient mutant of T4 bacteriophage requires its specific cofactor for the initial step of attachment to the host cell.

These experiments support the picture previously developed that virus invasion of host cells consists in an initial, reversible attachment whose properties are those to be expected from the operation of electrostatic binding forces. The step is followed by an enzymatic transformation which is irreversible, strongly temperature-dependent, and in the case of T1 virus, susceptible to inactivation by ultraviolet radiation.

The resistance of mutant cells to specific bacteriophages is of two types, depending on whether the first or second of these steps is blocked.