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It has been noted by the writers that in the preparation of germ-free filtrates, notably in working with the anaerobic organisms, those media which contained raw meat pieces added aseptically were superior to media to which meat cubes were added and which were subsequently sterilized by heat. It was believed-a belief later confirmed-that the presence of the unaltered proteins was chiefly responsible for the superiority of those media of which they were an integral part. However, the addition of meat cubes to previously sterilized media without contamination thereof is difficult and unpractical on a large scale. Therefore, in order to supply proteins unaltered by heat the juice obtained by subjecting finely ground and previously frozen and thawed raw meat was suggested as a substitute for meat cubes in the expectation that this juice could be sterilized by passing through a bacteria-retaining filter. Filtering of such a colloidal substance was found to be exceedingly difficult. Inasmuch as no literature seemed to be available which offered any assistance it is believed that a report of the technique now used by the writers will prove helpful to others. Preparation of the colloidal substances for the bacteria-retaining filter by preliminary passage through cotton, asbestos wool and filter paper does not appreciably enhance their filterability. However, by adding a sufficient amount of pulverized kieselguhr to the meat juice to make a very thin paste and pouring this mixture over a coarse filter paper, the filtrate so obtained will

THE FILTRATION OF COLLOIDAL SUBSTANCES THROUGH BACTERIA-RETAINING FILTERS