The relationship between 1-deoxynojirimycin (DNJ) content and alpha-glucosidase inhibitory activity in mulberry (Morus) leaves is discussed. Mulberry leaves were collected from the Center for Bioresource Field Science, Kyoto Institute of Technology, Kyoto, Japan on 19 May, 9 July, and 9 August, 2003. Mulberry leaves were extracted with 75% ethanol. The inhibitory activity for rat intestinal crude enzyme was measured using maltose. The content of DNJ in the extracts was measured using HPLC. The mean DNJ content in the 0.04-0.06% range was high in collected samples. The inhibitory activities in July and August were higher (P < 0.01) than in May, and the activity in July was higher (P < 0.01) than in August. A strong correlation (r = 0.901, r (2) = 0.811, n = 15) existed between DNJ content and alpha-glucosidase inhibition in leaves of Morus bombycis harvested in July. Similarly, correlation coefficients of the other mulberry varieties in July were higher than they were in May or August. The inhibitory activity and the DNJ content of Morus latifolia in August were lower than for any other mulberry variety. These results show that the high inhibitory cultivars harvested in July, except for M. latifolia, are more suited to products that contain high DNJ contents.