SummaryThe filtration end-point of the virus of vesicular stomatitis, or the average pore diameter of the finest membrane passing the virus, was found to be approximately 140 mμ. Two im-munologically distinct strains of the virus, the “Indiana” and the “New Jersey” maintained either in tissue culture or in mouse brain, were studied, and the filtration end-point was found to be the same irrespective of the source or serological type of the virus.Our results confirm those of Galloway and El ford, who found that the virus passes through collodion membranes which have an average pore diameter of 160 mμ. but is held back by those of 130 mμ. From their results they estimated the particle size of the virus to be between 70 and 100 mμ.