324 Activities and screening of Yemeni medicinal plant 16 plants have been determined for their antibacterial and antifungal activity by means of the agar diffusion method, antioxidant activity using scavenging activity of DPPH radical method and cytotoxic activity using the neutral red uptake assay. Furthermore, a phytochemical screening of the methanolic extracts was performed. The activities have been selected because of their great medicinal relevance. Within the recent years, infections have increased to a great extent and resistance against antibiotics becomes an ever-increasing therapeutic prob- lem (12). Because natural products of higher plants may give a new source of antimicrobial agents, there are many research groups that are now engaged in medicinal plants research (13–15). In the last years, interest in the antioxidant activity of plant extracts has become larger and very important (16–18) due to the fact that free radicals e.g. reactive oxygen species (ROS) can be responsible for various diseases, e.g. heart diseases, stroke, arteriosclerosis and cancer, as well as for aging process (19). Methods Plant Materials The plants were collected from different localities of Yemen in July 2005 and identified at the Pharmacognosy Department, Faculty of Pharmacy, Sana’a University. Part of the identification of the investigated plants was done by Priv.-Doz. Dr Peter Koenig, at the botanical garden, Ernst-Moritz-Arndt-University, Greifswald, Germany. Voucher specimens were deposited at the Pharmacognosy Department, Faculty of Pharmacy, Sana’a University. Extraction of Plant Material The air-dried and powdered plant materials (10 g of each) were extracted with 400 ml methanol (CH3OH) by using a Soxhlet apparatus for 8 h. The residue was dried over the night and then extracted with 250 ml water (H2O) by using a shaking water-bath at 70 C for 2 h. The extraction with water was repeated thrice. The water- filtrates were mixed together. The obtained methanolic and water extracts were filtered and evaporated by using a rotary evaporator and freeze dryer, respectively to give the crude dried extract. The dried extracts were stored at 20 C until used. Test Organisms The following microorganisms were used as test orga- nisms: Staphylococcus aureus (ATCC 6538), Bacillus subtilis (ATCC 6059), Micrococus flavus (SBUG 16), Escherichia coli (ATCC 11229), Pseudomonas aeruginosa (ATCC 27853) and Candida maltosa (SBUG). In addition, three multiresistant Staphylococcus strains namely, S. epidermidis 847, S. haemolyticus 535 and S. aureus North German Epidemic Strain (supply from the Institute of Hygiene of Mecklenburg-Vorpommern, Greifswald, Germany) were also applied as test organisms. Antimicrobial Assay The disc-diffusion assay (20) was used to determine the antimicrobial activity of the investigated extracts. Nutrient agar (OXOID LTD, Basingstoke, Hampshire, England) was prepared by dissolving of 27 g l 1 in water. The sterile nutrient agar was inoculated with microbial cells (200 ml of microbial cell suspension in 20 ml agar medium) and poured into sterile petri dishes. Sterile filter paper discs of 6 mm diameter (Schleicher and Schuell, ref. No. 10321260, lot. DG0274-1) were impregnated with 20 ml of the extract solution (equivalent to 4 mg of the dried extract). The paper discs were allowed to evaporate and after that placed on the surface of the inoculated agar plates. Plates were kept for 2 h in refrigerator to enable prediffusion of the extracts into the agar. Then, the plates were incubated overnight (18 h) at 37 C. In contrast, M. flavus was incubated at room temperature for 48 h and C. maltosa was incubated at 28 C for 48 h. Ampicillin, gentamicin and amphotericin B were used as positive control. Negative controls were performed using paper discs loaded with 20 ml of organic solvents (chloro- form and methanol). At the end of the incubation period the antibacterial activity was evaluated by measuring the inhibition zones (diameter of inhibition zone plus diameter of the disc). An inhibition zone of 14 mm or more was considered as high antibacterial activity. Determination of Antioxidant Activity Scavenging Activity of DPPH Radical In order to measure antioxidant activity, DPPH free radical scavenging assay was used. This assay measures the free radical scavenging capacity of the investigated extracts. DPPH is a molecule containing a stable free radical. In the presence of an antioxidant which can donate an electron to DPPH, the purple color which is typical for free DPPH radical decays, and the change in absorbency at 517 nm is followed specrophotometrically. This test could provide information on the ability of a compound to donate a hydrogen atom, on the number of electrons a given molecule can donate, and on the mechanism of antioxidant action. The method was car- ried out as described by Brand et al. (21). The methanolic and aqueous extracts were redissolved in methanol and 5% ethanol, respectively and various concentrations (10, 50, 100, 500 and 1000 mg ml 1) of each extract were used. The assay mixture contained in total volume of 1 ml, 500 ml of the extract, 125 ml prepared DPPH (1 mM in methanol) and 375 ml solvent (methanol or 5%