Studies on the pyrogen tolerance, reported formerly by M. Kobayashi (this Breviaria of vol. 47, p. 5 ξ, nos. 10 & 11) were more widely and precisely carried out. Result : When a certain intravenous dose of the filtrate of Escherichia coli culture. media, which had assayed biologically by means of investigation on the Schwartzman's phenomenon in rabbits and standardized uniformly, were given to rabbits once a day or, sometimes, with an interval for three days, the tolerance was proved acquired against the very pyrogen on animals after 5-7 days from the first injection, which lasted four weeks at least.