Feces are the product of host-gut microbiota communication, and they are often used for the study of gut microbiota-derived metabolites. To discover the new hostmicrobial AA-BA metabolites, we collected two pooled feces from 8 C57 male mice and 5 male healthy volunteers as typical samples and analyzed them using our developed polarity-switching MRM MS approach. A group of peaks with the same retention time (RT) and similar peak shape were screened from a pair of MRM(+) channels and corresponding MRM(-) channels as a potential AA-BA. Figure In the same way, a total of 118 AA-BA candidates involving 19 types of amino acid conjugations (glycine, alanine, serine, valine, leucine/isoleucine, cysteine, asparagine, aspartic acid, lysine, glutamine, glutamic acid, methionine, histidine, phenylalanine, arginine, tyrosine, threonine, and tryptophan) were detected, of which 58 AA-BAs were from mouse feces and 95 AA-BAs were from human feces (Figure ). The detailed information on these detected AA-BAs was presented in Table in Supporting Information-1. It should be noted that to our knowledge, 14 types of amino acid conjugations with host BAs (serine, valine, threonine, cysteine, asparagine, aspartic acid, lysine, glutamine, glutamic acid, methionine, histidine, arginine, isoleucine, and tryptophan) were previously unidentified. The molecular diversity of AA-BAs discovered in the human feces was higher than that in the mouse feces, which might be attributed to the higher complexity of the volunteers' diets than that of mice fed with a standard diet.