Tryptanthrin is a pharmacologically active compound in the anti-inflammatory herb Isatis tinctoria, with potent inhibitory activity on prostaglandin and leukotriene synthesis and on inducible NO synthase. The tryptanthrin content of five defined woad strains was analyzed in dependence of the time of harvest and post-harvest treatment. Tryptanthrin was determined by a validated ESI-LC-MS isotope dilution assay with d(8)-tryptanthrin as internal standard. The tryptanthrin concentration in freeze-dried leaf samples was low. Drying at ambient temperature led to a significant increase of tryptanthrin concentration, but the highest concentrations were found when leaves were dried at 40 degrees C. Tryptanthrin content in fermented woad leaves was below the limit of quantification. Tryptanthrin appears thus to be a product of post-harvest processes, but details of its formation remain to be elucidated.