A new monoterpene and a new guaianolide were isolated from the aerial parts of the Tibetan medicinal plant Soroseris hookeriana subsp. erysimoides (Asteraceae), in addition to (1R,4R,5R)-5-hydroxybornan-2-one 5-O-beta-D-glucopyranoside, beta-sitosterol, daucosterol, diosmetin, isoluteolin, p-methoxybenzoic acid, isovanillic acid, two phenylmethanol derivatives (vanilioloside and phenylmethanol glucopyranoside), and five guaianolides [3 beta,8 beta-dihyroxyguaia-4(15),10(14),11(13)-triene-12,6 alpha-olide, dentalactone, 10 alpha-hydroxy-8-deoxy-10,14-dihydrodeacylcinaropicrin, glucozaluzanin C and 8-epideacylcinaropicrin glucoside]. By a combination of spectroscopic methods (IR, EI-MS, 1H- and 13C-NMR, and DEPT), the structure of the new guaianolide was established as 3 beta,8 beta-dihydroxy-11 alpha H-guaia-4(15),10(14)-diene-12,6 alpha-olide, and that of the new monoterpene as (1R,4R,5R)-5-benzoyloxybornan-2-one. The antimicrobial activity of all isolates except the two sterols were measured using Escherichia coli, Bacillus subtilis, Staphylococcus aureus, Candida albicans, Aspergillus niger, and Trichophyton rubrum as test microorganisms. The new guaianolide was shown to be equally active (MIC: 50 micrograms/ml) against E. coli, B. subtilis and A. niger. The new monoterpene inhibited exclusively the growth of B. subtilis with MIC at 25 micrograms/ml. p-Methoxybenzoic acid and isovanillic acid were inhibitory against A. niger (MIC: 25 micrograms/ml), the latter being also active against B. subtilis with MIC at 25 micrograms/ml. The flavonoids diosmetin and isoluteolin almost equally inhibited the growth of B. subtilis (MIC: 25 micrograms/ml) and the human pathgenic fungus T. rubrum (MIC: 50 micrograms/ml).