The multiplication of Semliki Forest Virus (SFV) in myxoma subcut. tissue is described. In normal rabbits, intramusc. injn. of mouse brain SFV reduced the myxoma fatality from 100 to 62%; the fatality rate was only 25% when myxoma passed SFV was injd. with myxoma. In rabbits that had been infected 24 hrs. earlier with myxoma, the intramusc. injn. of the 45th passage of SFV in myxoma reduced the myxoma fatality rats from 100 to 51%.