Scutellarin was purified from the plant Erigeron breviscapus (Vant.) Hand.-Mazz. The activity against 3 strains of human immunodeficiency virus (HIV) was determined in vitro in this study. These were laboratory-derived virus (HIV-1IIIB), drug-resistant virus (HIV-1(74V)), and low-passage clinical isolated virus (HIV-1(KM018)). From syncytia inhibition study, the EC50 of scutellarin against HIV-1IIIB direct infection in C8166 cells was 26 microM with a therapeutic index of 36. When the mode of infection changed from acute infection to cell-to-cell infection, this compound became even more potent and the EC50 reduced to 15 microM. This suggested that cell fusion might be affected by this compound. By comparing the inhibitory effects on p24 antigen, scutellarin was also found to be active against HIV-1(74V) (EC50 253 microM) and HIV-1KM018 (EC50 136 microM) infection with significant difference in potency. The mechanism of its action was also explored in this study. At a concentration of 433 microM, scutellarin inhibited 48% of the cell free recombinant HIV-1 RT activity. It also caused 82% inhibition of HIV-1 particle attachment and 45% inhibition of fusion at the concentrations of 54 microM. In summary, scutellarin was found to inhibit several strains of HIV-1 replication with different potencies. It appeared to inhibit HIV-1 RT activity, HIV-1 particle attachment and cell fusion. These are essential activities for viral transmission and replication.