The Rothamsted culture of tobacco necrosis virus contains two serologically unrelated viruses one of which, called the "satellite virus" (SV), causes no lesions and multiplies detectably only when the other (TNV) is present. It decreases the size of necrotic local lesions formed by TNV. Inocula containing both viruses gave the same, fewer or more lesions than inocula containing only TNV; the results depended on the ratio of the two viruses in the mixed inoculum, on the species and age of the test plants and, particularly, on the temperature at which the plants were kept after the inoculation. The concentration of TNV decreased when SV was also present; the extent of the decrease depended on the ratio of the two viruses in the inoculum and other factors. Inoculation with SV 5 days before inoculation with TNV still decreased lesion size and led to the multiplication of SV. Phenol-disrupted SV affected lesion size only when inoculation with TNV followed within a few hours, but when inoculation with TNV was delayed by up to 2 days, SV sometimes multiplied detectably. SV has the smallest particle reported for any plant virus; diameter c. 17 m[mu] and weight equivalent to a molecular weight of about 1.9 x 106, of which 20% is nucleic acid. It is exceptionally stable, retaining infectivity after 17 years at 3[degree] and after heating for 10 min. at 90[degree]. TNV was much more susceptible to inactivation by heat and by ultraviolet light than SV. The two differed greatly in electrophoretic mobility and were readily separated from mixed preparations by electrophoresis.