1. The inactivation of antistaphylococcus bacteriophage suspended in infusion broth at pH 7.6 and 22°C. by HgCl2 proceeds according to the equation dP/dt = k [HgCl2] [Po – Pi] over the range studied.
2. This inactivation can be reversed by precipitation of Hg++ with H2S. In the present experiments the inactivation was carried out until only some 5 per cent of the initial phage remained active. After reactivation the [P] had increased to 100 per cent of the initial [P].