Forty preparations of the extracts from 28 kinds of Asian herbs were tested for ability to inhibit the activities of murine retroviral reverse transcriptase and human deoxyribonucleic acid (DNA) polymerases. Among the 40 extracts, 35 inhibited reverse transcriptase activity and 29 inhibited DNA polymerase α activity. The inhibitory potencies of these extracts were expressed as the 50% inhibition concentrations (IC₅₀), at which the enzyme activities were inhibited by 50%. Very strong inhibitions were observed with the extracts from Millettia pachycarpa (Leguminosae) and Mallotus apelta (Euphorbiaceae) as shown by their low IC₅₀ values for reverse transcriptase (0.4-0.5 μg/ml) and DNA polymerase α(0.9-1.4 μg/ml). Enzyme kinetic analysis revealed that the mode of inhibition of reverse transcriptase by these two extracts was competitive with respect to the template·primer [poly(rA)·oligo(dT)] and noncompetitive with respect to deoxythymidine triphosphate (dTTP) substrate. Besides reverse transcriptase and DNA polymerase α, DNA polymerase I and ribonucleic acid (RNA) polymerase from E. coli were inhibited by these two extracts. These results indicate that the herb extracts contain as yet unidentified substance(s) which inhibit the activities of reverse transcriptase and cellular DNA polymerases.