Extracts of Chilean medicinal plants were evaluated in vitro for their activities against influenza virus proliferation in MDCK cells. The most potent extract obtained was from Muehlenbeckia hastulata (Polygonaceae), known as Quilo in Chile, from which three active principles were isolated and identified as pheophorbide a (1), hypericin (2) and protohypericin (3). Minimum inhibitory concentration (MIC) values of 42 ng/ml for compound 1, 2.1 ng/ml for compound 2 and 1.5 ng/ml for the authentic hypericin were determined by using an endpoint assay which comprises pre-incubation of serially diluted specimens with a given amount of the influenza virus, incubation of the pre-incubated virus/specimen with MDCK cells and determination of the hemagglutination (HA) titer of the culture supernatant. Compound 3 was easily converted to 2 on exposure to visible light and, in due course, showed an anti-influenza virus activity (3.1 ng/ml) similar to 2. Although compounds 1-3 were previously isolated from other plants, this is the first report of their isolation from M. hastulata. The high content of 1 (0.06% dry weight of whole plant) is noteworthy. In addition, this is the first report on the isolation of compounds 2 and 3 from a plant other than the genus Hypericum.