Different components of brome mosaic virus (BMV) RNA, either separate or mixed, stimulated similarly the activity of soluble BMV-RNA polymerase. When unfractionated RNA was heated, the various components disappeared in order of molecular size (the largest fastest), and the infectivity remaining depended on the concentration of the three largest components. The enzyme-stimulating activity, however, decreased more slowly and was still 40% of that of unheated RNA when only the small BMV-RNA component (0.28 × 106 daltons) remained partly undegraded. We suggest reasons why each component of BMV-RNA can serve as template, yet in the complete mixture the smallest appears by far the most active