The purpose of this work was to identify the principal human cytochrome P450s (CYPs) involved in the metabolism of the retinoic acid (RA) isomers, 9-cis- and 13-cis-RA, by using a combination of techniques including human liver microsomes (correlation of activity and inhibition), and lymphoblast microsomes expressing a single CYP. Concerning the 9-cis-RA, 4-OH- and 4-oxo-9-cis-RA were formed with human liver microsomes, and their formation correlated with activities linked to CYPs 3A4/5, 2B6, 2C8, 2A6, and 2C9. The use of lymphoblast microsomes expressing a single human CYP identified CYPs 2C9>2C8>3A7 as the most active in the formation of 4-OH-9-cis-RA. With regard to 13-cis-RA, specific P450 activities linked to CYPs 2B6, 2C8, 3A4/5, and 2A6 were correlated with the formation of 4-OH- and 4-oxo-13-cis-RA. Microsomes expressing a single CYP identified CYPs 3A7, 2C8, 4A11, 1B1, 2B6, 2C9, 2C19, 3A4 (decreasing activity) in the formation of 4-OH-13-cis-RA. The use of CYP-specific inhibitors in human liver microsomes disclosed that the formation of the 4-OH-9-cis-RA was best inhibited by sulfaphenazole (72%) and quercetin (66%), whereas ketoconazole and troleandomycin inhibited its formation by 55 and 38%, respectively; the formation of 4-OH-13-cis-RA was best inhibited by troleandomycin (54%) and ketoconazole (46%), whereas quercetin was a weak inhibitor (14%). In conclusion, adult human CYPs 2C9, 2C8, 3A4 have been identified as active in the 9-cis-RA metabolism, whereas CYPs 3A4 and 2C8 were active in 13-cis-RA metabolism. The fetal form CYP3A7 was also identified as very active in either 9-cis- or 13-cis-RA metabolism. The role of these human CYPs in the biological response or resistance to RA isomers remains to be determined.