As part of our screening of anti-AIDS agents from natural sources, ethanolic and aqueous extracts of 15 medicinal plants widely used in the folk medicine of the Iberian Peninsula were evaluated in vitro. Most of the extracts tested were relatively nontoxic to human lymphocytic MT-2 cells, but only the extracts of Tuberaria lignosa and Sanguisorba minor magnolii exhibited anti-HIV activity in an in vitro MTT assay. The aqueous extracts of these plants showed inhibitory effects against HIV-1 induced infections in MT-2 cells at concentrations ranging from 12.5 to 50 microg/ml and 50 microg/ml, respectively. Both extracts showed no appreciable cytotoxicity at these concentrations.