Some properties of an unidentified, filterable, infectious agent found in the blood and organs of mice with several neoplastic diseases are described. It is not the LDH factor. Its only measurable effect was to cause a 2-4 fold increase in spleen weight within a week following intraperitoneal injection. All mouse strains tested were susceptible. Its activity was suppressed by terramycin. It resisted sedimentation at 50,000 x g (1 hour). Millipore filtration indicated that it was between 100 and 300 m[mu]. Chromatography (ECTEOLA) suggested that it consisted of 2 entities. Its relation to Eperythrozoon coccoides is discussed.