RNA-based amplification systems that have been recently described are dependent upon the presence of more than one enzyme. In an attempt to minimize the number of polymerases required for efficient amplification, we have studied the template specificity of bacteriophage T3 RNA polymerase. A synthetic bacteriophage T3 promoter was covalently attached to an RNA template. The T3 promoter-RNA complex was found to be selective for its native polymerase, and dependent upon the presence of all four ribonucleoside precursors. The product of the RNA-directed transcription is complementary to the initial template.