In the course of a screening of Iraqi plants for antimicrobial agents, ethanolic extracts of the seeds of Peganum harmala (fam. Zygophyllaceae) were found to be particularly potent and showed a broad spectrum of activity against the screening organisms. Subsequent fractionation of the defatted seeds showed the bioactivity was restricted to the non-phenolic tertiary alkaloid fraction. The active fractions were analyzed by HPLC, which identified alkaloids including harmaline, harmine, vasicine, and deoxyvasicinone. Samples of the various alkaloids were subjected to in vitro antibacterial evaluation, and results showed the broad spectrum activity of Iraqi P. harmala is primarily due to harmine, while other alkaloids (harmaline, vasicine, deoxyvasicinone) are inactive at the top level tested (100 mcg/ml). P. harmala has an ancient reputation as an antiseptic and for treatment of skin diseases, and the smoke of the plant has been used as a disinfectant, but the active agent (evidently harmine) has not previously been identified. The finding that such a relatively simple and well-known β-carboline alkaloid possesses antimicrobial activity and that this is shared by related simple bases is of some interest and recalls the previous finding that the canthin-6-one series also possessed activity of this type.