1. The complete inactivation of antistaphylococcal phage by HgCl 2 (2.8 per cent for 216 hours) can be reversed by precipitation of Hg ++ with restoration of the phage to its original titre. 2. This behavior seems more compatible with the known properties of certain enzymes than with those of living protoplasm.

1. The inactivation of antistaphylococcus bacteriophage suspended in infusion broth at pH 7.6 and 22°C. by HgCl 2 proceeds according to the equation dP/dt = k [HgCl 2 ] [ P o – P i ] over the range studied. 2. This inactivation can be reversed by precipitation of Hg ++ with H 2 S. 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 ].