A method of estimating the purity of preparations of elementary bodies of vaccinia is described. It depends on the comparison of the number of infective units of virus in a given material with the number of elementary bodies. The latter figure is estimated from the dry weight of the preparation by means of a calculated value for the weight of a single dehydrated elementary body.
Values for the ratio of infective units of vaccine virus to elementary bodies varied between 1 : 2.4 and 1 : 9.2 in seven consecutive experiments; the average was 1:4.2. These ratios indicate a high degree of purity of the preparation. Moreover, they indicate that a relatively high percentage of the elementary bodies in the preparations was infective.