1. In the case of the staphylococcus and antistaphylococcus phage studied, with the total volume of the mixture being kept constant, there exists a definite quantitative relationship between CPhage and the time required to reduce a particular concentration of growing phage-susceptible bacteria to an arbitrary turbidity end-point.
2. This relationship furnishes a basis for the quantitative estimation of bacteriophage. A method is described having an accuracy within ±5 per cent.