The principal objective of this research was to determine the nature and extent of the effect of water‐softening precipitation processes on bacterial viruses. The virus used in this study was the T2 bacteriophage. Esch. coli, strain B, was used in a pure culture as the host bacterium for quantitative determinations of the virus concentration. Stock solutions of coagulants were prepared from reagent grade chemicals. Synthetic water samples, containing known amounts of calcium, magnesium, alkalinity, carbon dioxide, and total hardness in the range of 100‐500 mg/l were prepared and placed in each of five 2,000‐ml reaction beakers. A sixth beaker, containing the same volume of distilled water adjusted to pH 7.0 with a hydrated lime solution, served as a control. The six beakers were placed under a standard jar test machine with a variable‐speed mixer. Appropriate volumes of stock bacteriophage suspension were diluted and added to each of the beakers. Immediately after the initiation of rapid mixing (100 rpm), the required amounts of softening chemicals were added, and rapid mixing was continued for one minute after addition of the chemicals.