15 HIV-infected patients on stable antiretroviral therapy including etravirine at the dosage of 200 mg twice daily during at least 4 weeks will be enrolled. After their inclusion in the study, patients will receive Echinacea purpurea root (500 mg every 8 hours) in addition to their antiretroviral treatment, from day 1 until two weeks later (day 14). On days 0 and 14, blood samples will be drawn immediately before and 1, 2, 4, 6, 8, 10 and 12 hours after the administration of an etravirine dose, and etravirine concentrations in plasma will be determined by high performance liquid chromatography using a validated method.