During the course of our studies on the metabolism of iridoid glycosides by human intestinal bacteria, we found that geniposide (1) and gardenoside (4) were transformed to new nitrogen-containing compounds, genipinine (3) and gardenine (6), respectively, along with the known aglycones. Although the amounts of new metabolites were somewhat lower than those of the aglycones, they were quantitatively analyzed by means of liquid chromatography/mass spectrometry (LC/MS). Of 25 strains of human intestinal bacteria, Peptostreptococcus anaerobius, Klebsiella pneumoniae, Fusobacterium nucleatum, and Bacteroides fragilis ssp. thetaotus produced appreciable amounts of 3, while a bacterial mixture of human feces produced 10 times or more higher amounts of 3, as compared to the individual strains.