In the search for novel anti-human immunodeficiency virus type I (anti-HIV-1) agents from natural sources, 49 MeOH extracts of Korean plants were screened for their inhibitory effects against RNA-dependent DNA polymerase (RT) and ribonuclease H (RNase H) activities of HIV-1 reverse transcriptase and HIV-1 protease, and anti-HIV-1 activity. Regarding the HIV-1 reverse transcriptase, Agrimonia pilosa (whole plant), Cornus kousa (stem and leaf), Limonium tetragonum (root) and Mallow japonicus (stem) showed significant inhibitory activity on RT activity with 50% inhibitory activity (IC50) of 8.9, 6.3, 7.5 and 11.9 mug/mL, respectively, whereas Agrimonia pilosa was also active against RNase H activity (IC50 = 98.4 mug/mL). Four plants, namely Agrimonia pilosa (whole plant), Atractylodes japonica (root), Clematis heracleifolia (whole plant) and Syneilesis palmata (whole plant), were appreciably active (< 35%) against recombinant HIV-1 protease at a concentration of 100 mug/mL. Crinum asiaticum var. japonicum (root) showed significant anti-HIV-1 activity (ED50 = 12.5 mug/mL) with a favourable SI value of 16. Copyright (C) 2001 John Wiley & Sons, Ltd.