The quaternary benzo[c]phenanthridine alkaloids (QBAs) are an important subgroup of plant secondary metabolites. Their main representatives, sanguinarine (SG) and chelerythrine (CHE), have pleiotropic biological effects and a wide spectrum of medicinal applications. The biotransformation of SG and CHE has only been partially studied while subsequent oxidative transformation of their dihydro derivates, the main metabolites, is practically unknown. The aim of this study was to characterize the biotransformation of CHE and dihydrochelerythrine (DHCHE) in detail, with respect to their more extensive biotransformation than SG. Phase I as well as phase II biotransformation of both compounds was examined in human hepatocyte suspensions. Liquid chromatography with electrospray-quadrupole time-of-flight mass spectrometry (LC-ESI-QqTOF MS) was used for analysis of the metabolites. Using the LC-ESI-QqTOF MS method, we analyzed and then suggested the putative structures of 11 phase I and 5 phase II metabolites of CHE, and 11 phase I and 6 phase II metabolites of DHCHE. For the most abundant metabolites of CHE, DHCHE and O-demethylated DHCHE, their cytotoxicity on primary cultures of human hepatocytes was analyzed. Both metabolites were nontoxic up to 50μM concentration and this indicates decreasing toxic effects for CHE biotransformation products, i.e. DHCHE and O-demethylated DHCHE.