Summary We have investigated the interaction between a number of 14 mers phosphorothioate oligonucleotides and HIV-1 reverse transcriptase. Two methods were used to measure the affinity of the analogs for the enzyme. In the first, the oligonucleotide or its duplex with Poly(rl) were used as inhibitors of the enzyme using Poly(rA).(dT) 14 as template primer. In the second, the oligonucleotides or their duplexes were used to displace a fluorescent template.primer complex of known affinity from its binding site on reverse transcriptase. The two methods gave the same relative order of affinitiy. Phosphorothioate oligodeoxyribonucleotides had a much higher affinity than oligo(dC) 14 and it was increased on hybridization. Quantitatively similar results were obtained for S(dC) 14 or its analog with bases in the a-configuration. Of the analogs tested, only S(dC) 14 showed priming activity.