ABSTRACT Even though pharmacological industries have produced a number of new antibiotics in the last three decades, resistance to these drugs by bacteria has increased. Hence the present study was carried out with an objective to investigate the antibacterial potential of leaf and flower extracts of Parthenium hysterophorus L. The aim of the study is to assess the antibacterial activity and to determine the zone of inhibition of methanol and acetone leaf and flower extracts on some bacterial strains. The in vitro tests were conducted for their antibacterial activities against two Gram positive i.e., Bacillus subtilis, Staphylococcus aureus and two Gram negative i.e., Escherichia coli, Pseudomonas aeruginosa bacteria with agar disc diffusion method. Zone of inhibition of leaf and flower methanol and acetone extracts were recorded and compared with standard drug, streptomycin. The results showed that there was remarkable inhibition of the bacterial growth against the tested organisms. The bacterial growth inhibitory activity may be due to the presence of various secondary metabolites present in the tested plant. Hence, it is suggested that the plant can be further investigated to discover the bioactive compound that may serve as lead compound in the development of new pharmaceutical research activities. Key words: Parthenium hysterophorus, Antibacterial activity, Agar disc diffusion method, Methanol and acetone solvents, leaf and flower extracts. REFERENCES Afolayan, A. J. Pharm. Biol., 2003, Observed antibacterial activity of B.orellana leaf extract of various solvent. 41, 22-25. Auld BA, Hooking J, Mc Fadyen RE, 1983. Analysis of the spread of tiger pear and Parthenium hysterophorus weed in Australia. Australian Weeds;Vol 2: pp 56-60. Bhat, S., Mercy Lobo, S., Chethan Kumar, K.V., Sukesh and Chandrashekar, K.R. J. of Phytol., 2009, Anti-fungal activity of Parthenium hysteroporus. 1(6), 469– 474. Bhatt, J.R., Singh, J.S., Singh, S.P., Tripathi, R.S., and Kohli, R.K, 2012. Invasive Alien Plants: An Ecological Appraisal for the Indian Subcontinent (CABI). Butler, J.E., 1984. Longevity of Parthenium hysterophorus L. seed in the soil. Australian Weeds, Vol 3, pp 6-6 Choudhury pradeep kumar, Dinda subas Chandra, Dash Santhosh kumar ,2012. Anti microbial activity of ficus racemosa L.and cissampelos pareira L.var. hirsute (DC) forman.,against E.coli, P.aurogninosa, S.aureus, B.subtilis ,International Research journal of pharmacy. , 3(12), pp 151. Cowan, M.M. Clin. Microbiol. Rev., 1999, plant products as anti microbial agents, clinical. 12(4), 564-582. Jayachandra, 1971. Parthenium hysterophorus weed in Masore state and its control. Curr. Sci;40(21):568-569. Kohli, R.K., Batish, D.R.,Singh, H.P. and K.S. Dogra, 2009. Ecological Status of Some Invasive Plants of Shiwalik Himalayas in Northwestern India in Invasive Plants and Forest Ecosystem, Kohli, R.K., S. Jose, D.R. Batish and H.P. Singh (Eds.)., CRC/Taylor Press, Netherlands, ISBN: 978-1-4200-4338-9, pp 143-155. Krishnamurthy K, Ramachandra Prasad TV, Muniyappa TV, 1975.Agriculture and health hazards of Parthenium hysterophorus. Curr. Res; Vol 4: pp 169-171. Parekh, J., Darshana, J. and Chanda, S. Turk. J. Biol., 2007. Efficacy of aqueous and methanol extracts of antibacterial activity. 29,203-210. Parsons, W.T., and Cuthbertson, E.G, 2001. Noxious Weeds of Australia (CSIRO Publishing). Saeed S, Naim A and Tariq P, 2007.A study on prevalance of multi-drug-resistant Gram-negative bacteria. Int. J. Biol. Biotech., 4(1): 71-74. Satish S, Raghavendra MP, Mohana DC and Raveesha KA , 2008. Antifungal activity of a known medicinal plant Mimusops elengi L. against grain moulds. J. Agri. Technol. 4(1), 151-165. Sofowara, A. J. of Ethanopharmacol.,1982, Medicinal plants and traditional medicinal in Africa.,antifungal metabolites from plants. 100, 80-84. Sushilkumar, 2012. Current spread, impact and management of parthenium weed in India. Int. Parthenium News, 5, pp 1-6.