A filterable agent resembling the viruses is described. It was encountered in sarcoma 39, a propagable neoplasm of the white rat, and has now been maintained in this species for 28 passages over a period of some 7 months without appreciable loss in virulence. Its chief effect is the production of large abscesses in an animal species comparatively resistant both to viral diseases and suppuration. The white mouse is more susceptible than the white rat, the rabbit less so, and the guinea pig highly resistant.
The agent has been repeatedly recovered from sarcoma 39 treated in special ways, but under the ordinary circumstances of routine transplantation it does not manifest itself. As yet there is no certainty on where it came from or how it maintains itself under natural conditions.