The aqueous lemon balm extract is fully active against HIV-1 mutants with reduced enfuvirtide sensitivity, but looses potency against surface-bound virions. HIV-1NL4-3 wildtype, HIV-1NL4-3 Env (V38A) and HIV-1NL4-3 Env (X23) virions were exposed to lemon balm extract at concentrations from 0.00001 to 1% for 1 h at 37°C and subsequently added to TZM-bl cells. Alternatively, TZM-bl cells were pretreated with enfuvirtide at concentrations ranging from 0.0032 to 10 μM for 15 min and then inoculated with HIV-1NL4-3 wildtype and mutants. TZM-bl cells were washed the following day. 48 h post infection cells were lysed and the luciferase activity was quantified. Each experiment was performed in triplicate, and four independent experiments were conducted. IC50 values for HIV-1NL-43 wildtype infections were arbitrarily set to 1 and a factor of difference was calculated for the mutant strains, corresponding to the degree of resistance, for each experiment. Shown are arithmetic means ± standard error of the mean (SEM) of the degree of resistance from four independent experiments. (B) HIV-1 GFP reporter viruses pseudotyped with JR-FL Env were either pretreated with lemon balm extract for 1 h at 37°C and then added to TZM-bl cells or added to target cells simultaneously with the extract. (C) Alternatively, virions were either prebound to cells for 2 h at 4°C, washed, exposed to extract and then shifted to 37°C, or added to cells simultaneously with the extract at 37°C. (B, C) The experiments shown are representative for 2–4 independent experiments and the arithmetic mean ± SD (n = 3) is given.