Extract treatment enhances the density of HIV-1 particles. Purified HIV-1NL4-3 virions were exposed either to aqueous lemon balm extract (1%), solvent (1%), or Triton X-100 (0.5%) for 1 h at 37°C and subsequently ultracentrifuged in a sucrose-density equilibrium gradient. Gradient fractions were collected from top to bottom and fractions analyzed by anti-p24 ELISA. Results from two independent experiments are shown. The inscription for the x-axis of (B, experiment 2) applies also to the x-axis of (A, experiment 1).