A competitive ELISA technique using digoxin-specific antibody has been developed to determine digoxin and its related compounds in hairy root cultures of Digitalis lanata. The ELISA could detect 0.2-2 nM digoxin and closely related cardenolides. Hairy roots cultured in the dark accumulated very small amounts of cardenolides (0.02-0.07 micrograms/g dry wt), while the content of cardenolides in green hairy roots cultured in the light was increased maximally 600-fold (16.5 micrograms/g dry wt) compared to those in the dark.