The 2'-substituted analog of poly(inosinic acid) ((I)n), poly(2'-fluoro-2'-deoxyinosinic acid) ((dIfl)n) served as an effective template for the RNA-directed DNA polymerase (reverse transcriptase) from Moloney murine leukemia virus. When assayed under the same conditions, the parent compound (I)n showed little, if any, template activity. In the presence of other templates, i.e. poly (2-O-methylcytidylic acid), (dIfl)n could also assume the role of primer for the reverse transcriptase reaction, whereas, again, (I)n failed to do so.