A liquid chromatography-tandem mass spectrometry (LC-MS-MS) method was developed to detect tumor-promoting diterpene esters of the tigliane and ingenane types within plant extracts. Fractionation on a C18 high-performance liquid chromatography (HPLC) column was followed by MS-MS-multiple reaction monitoring (MRM) using the precursor-->product ion pairs of m/z 311-->293 and 293-->265 for phorbol esters. The ion pairs m/z 313-->295 and 295-->267 were used for ingenol and deoxyphorbol esters. In a second run, the characteristic ions at m/z 311 and 313 were followed in precursor ion scan mode. These quasi-molecular ions were utilized to obtain full scan spectra of the compounds in product ion scan mode. Due to its selectivity, the present on-line method can be applied for plant cultivar selection and plant product control without time-consuming extraction procedures and complex bioassays.