A filterable agent was isolated from 7 cases of spontaneous rabbit pox by the intratesticular injection in rabbits of a variety of tissues. The virus was transmitted for 15 consecutive testicle to testicle passages in rabbits by Berkefeld V filtrates of testicular tissue emulsions. Unfiltered emulsions were more potent than filtrates. The virulence of the virus was maintained by ice box storage of infected tissues for as long as 127 days. The pathogenic properties of the virus persisted under conditions of animal passage for the 9 months from January to October, 1933, covered by these studies. The reaction produced in rabbits by the inoculation of tissue-virus emulsions was of two principal types, the first of which has been described and analyzed in the present paper. In the case of intratesticular injections, particularly of testicular tissue inocula, an acute fulminating and rapidly fatal condition regularly developed. The outstanding features were a massive hemorrhagic orchitis with marked scrotal edema, fever, and death within a week. The second type of reaction which is taken up in the next paper of this series (1) was observed under conditions in which the animal survived a week or longer. Occasional examples occurred in rabbits of the so called intratesticular series. The reaction was characterized by the development of a disease syndrome with a diversity of clinical manifestations which, it may be stated here for the sake of continuity, was indistinguishable from spontaneous rabbit pox.