The beta-carboline alkaloids of Peganum harmala L were extracted through a bioassay-guided fractionation and their antimicrobial activities were investigated. Results revealed significant differences (P>0.05) between compounds depending on the microorganism tested and the application method. When examined individually, harmine was the most effective against Proteus vulgaris, Bacillus subtilis and Candida albicans where inhibition zones ranged between 21.2 and 24.7 mm. Potentiality of the alkaloids was increased when applied as binary mixtures suggesting a kind of synergistic interaction with inhibition zones reaching 31.5 mm with the total alkaloidal extract. We recommended the use of such compounds as new antimicrobial biorationals. (C) 2010 Elsevier B.V. All rights reserved.