Diabetes mellitus is one of the most prevalant diseases of adults. Agents with alpha-glucosidase inhibitory activity have been useful as oral hypoglycemic drugs for the control of hyperglycemia in patients with type 2; noninsulin-dependent. diabetes mellitus (NIDDM). Investigation of some medicinal herbs: Urtica dioica, Taraxacum officinale, Viscum album, and Myrtus conununis with alpha-glucosidase inhibitor activity was conducted to identify a prophylactic effect for diabetes in vitro. All plants showed differing potent alpha-glucosidase inhibitory activity. However. Myrtus communis strongly inhibited the enzyme (IC50 = 38 mug/mL). The inhibitory effect of these plants and some common antidiabetic drugs against the enzyme source (baker's yeast, rabbit liver, and small intestine) were also searched. Approximately all inhibitors used in this study showed quite different inhibitory activities, according to alpha-glucosidase origins. Furthermore, subsequent separation of the active material from Myrtus communis by HPLC showed that only one fraction acted as an alpha-glucosidase inhibitor.