Viability of the virus of foot-and-mouth disease was satisfactory in an acid sol., good in weakly alkaline, but very poor in strongly alkaline sol. When a weakly alkaline sol. was rendered strongly alkaline and at once neutralized, the effect of the alkali, which lasted only for seconds, impaired the virus. When a weakly alkaline sol. was acidified and at once neutralized, the acid environment, lasting only for a moment, destroyed the virus entirely, in spite of an acid reaction which, under other circumstances, is favorable. The fact that the acid virus-sol. can no more be brought back to the pH for its opt. activity without losing its infectiousness is called the "realkalizing effect." The viruses of foot-and-mouth disease and of stomatitis vesicularis are shown to be not identical, but closely related. The latter is insensitive to acidification and realkalization and is destroyed only in an acid medium.