The agent transmitting leucosis readily passed all types of silicious filters. Filtration is particularly successful when the plasma is freed from particles and substances that would otherwise obstruct the pores of the filter. Filtration through fine filters seems to be facilitated by preceding filtration through coarse filters.
A comparison of the periods of incubation of leucosis produced by unfiltered plasma and plasma passed through silicious filters shows that as a result of filtration, the incubation periods are somewhat prolonged. This suggests a slight or moderate decrease in the concentration of the transmitting agent in the plasma caused by filtration.
Filtration tests through collodion membranes indicate that the agent transmitting leucosis is much smaller than the virus of bovine pleuropneumonia (250 mµ) and that it approximates the size of bacteriophage.