AbstractThree major RNA species were resolved from TBRV-G, an isolate of the potato bouquet str. from Germany. Assuming that all 3 were linear single-stranded molecules, mol. wt. estimates were 2.5 X 106 (RNA-1), 1.5 X 106 (RNA-2) and 0.5 X 106 (RNA-3). Preparations of RNA-1 were moderately infective to Chenopodium amaranticolor, RNA-2 had little infectivity and RNA-3 did not induce any lesions. Mixtures of RNA-1 + RNA-2 were more infective than RNA-1 alone, but the infectivities of RNA-1 or RNA-1 + RNA-2 were greatly reduced by the addition of RNA-3. RNA extracted from lesions produced by RNA-1 infections contained both RNA-1 and RNA-2 but much less RNA-3 than the original isolate, and when this RNA-1 was used as inoculum only RNA-1 and RNA-2 were produced. From these and other results it was concluded that TBRV resembles raspberry ringspot virus in having 2 essential RNA components. RNA-3 appeared to be unnecessary for multiplication of TBRV and did not spontaneously recur in cultures containing only RNA-1 and RNA-2. It would seem that RNA-3 from TBRV is analogous to the satellite RNA found in some cultures of tobacco ringspot virus, although its mol. wt. is c. X 6 greater. It is not clear how RNA-3 inhibits lesion formation, but this ability is removed by UV irradiation.