a suspension of bacteriophage of unknown titer is first subjected to serial dilution, as illustrated in Figure 7.1. A small portion (0.1 mL) of the suspension is added to 9.9 mL dilution fluid, for a 100-fold dilution. A small portion (0.1 mL) of this dilution, in turn, is added to a fresh tube containing 9.9 mL dilution fluid, for another 100-fold dilution. At this point, the original phage suspension has been diluted by a factor of 10,000. the process is repeated two more times, to a fourth tube where the dilution factor is 108. Dilutions, of course, do not always have to be 1:100; other dilutions are perfectly acceptable, as long as the dilution factor is duly noted, as this is needed to obtain the titer of the original stock. In provide a 1:10 dilution, bringing the overall dilution factor to 109. The need for large dilutions is because phage titers can be quite high.