Plant β-glucosidases (β-D-glucoside glucohydrolases, E.C. 3.2.1.21) are known to function in defense in cyanogenic systems, and may also function in the metabolism of phytohormone glucosides and glucosides of other secondary plant products. Maize (Zea mays L.) β-glucosidase is a homodimer of 60 KD monomers and occurs in the plastid. Numerous glycosides were tested as substrates and K_m's and V_m's were determined. Various compounds were also tested as inhibitors and K_i's and/or K_i’ 's were estimated. 4- methylumbelliferyl-β-D-glucoside was the best substrate (K_m=0.14 mM) for which kinetic data were obtained. Monosaccharides were poor inhibitors. The best competitive inhibitors were D-gluconic acid lactone, dhurrin, and 2,4- dihydroxy-7-methoxy-2H-1, 4-benzoxazin-3(4H)-one (DIMBOA) (K_i's< 1 mM). The enzyme had broad substrate specificity and could cleave many glycosides with hydrophobic aglycones. One major substrate, the hydroxamic acid 2,4-dihydroxy-7- methoxy-1, 4-benzoxazin-3-one-8-D-glucopyranoside (DIMBOA-glc), was found in methanolic maize extracts. Increasing DIMBOA levels are associated with increasing resistance to several pests. The highest levels of enzyme activity and hydroxamic acids along K55 and H95 shoot length were found near the node. Tissue distribution of hydroxamic acids and β-glucosidase activity was also determined. In K55, both were found in the stele of the mesocotyl, the young leaves, and associated with the vascular bundles of the coleoptile. A major function of the enzyme is to mediate insect and pathogen resistance via the release of the toxic aglycone DIMBOA.