A ten-fold dilution reduces the concentration of a solution or a suspension of virus by a factor of ten that is to one-tenth the original concentration. A series of ten-fold dilutions is described as ten-fold serial dilutions. In this manual, ten-fold serial dilutions are used in titrations of a suspension of Newcastle disease virus to establish the infectivity titre. They are carried out in small sterile test tubes. These tubes are usually made of glass and it is preferable if they have fitted lids to minimize the risk of contamination during the dilution.