The multistep synthesis and negative ion-ESI fragmentation pattern of [methyl-D(3)](2)hypericin (1-D(6)) is described. The application of 1-d(6) as internal standard for the quantification of hypericin (1) in the ng mL(-1) range in human plasma by isotope-dilution LC-MS is demonstrated. The hypericin-containing plasma samples are spiked with 1-D(6), deproteinized and extracted with ethyl acetate. The extracts are injected into a HPLC-ESI-ion-trap system and the mass-separated negative ions from 1 and 1-D(6) are analysed. From their intensities linear standard curves over the concentration range from 1 to 10 ng mL(-1) are obtained. Accuracy, precision and recovery are discussed.