Previously unknown metabolites from the two macrolide immunosuppressants rapamycin (sirolimus) and SDZ RAD [40-O-(2-hydroxyethyl)rapamycin] obtained after in vitro incubation with human liver microsomes have been purified. Structure elucidation was performed by nanoelectrospray ionization tandem mass spectrometry applying low energy collision activated dissociation. This ionization method is, as shown here, a powerful tool to determine metabolic pathways by analysis of even low abundance products. Product ion spectra of the isolated metabolites indicate a new kind of biotransformation reaction for rapamycin and SDZ RAD. The proposed metabolic pathway starts with an ester hydrolysis which leads to a ring-opened structure. A dehydration on C33-C34 and a supplementary hydrogenation at C33-C34 result in a structure similar to the ring-opened isomer with an single bond at C33-C34.