Genetic variations in cytochrome P450 2C19 (CYP2C19) contribute to interindividual variability in the metabolism of therapeutic agents such as clopidogrel. Polymorphisms in CYP2C19 are associated with large interindividual variations in the therapeutic efficacy of clopidogrel. This study evaluated the in vitro oxidation of clopidogrel by 21 CYP2C19 variants harboring amino acid substitutions. These CYP2C19 variants were heterologously expressed in COS-7 cells, and the kinetic parameters of clopidogrel 2-oxidation were estimated. Among the 21 CYP2C19 variants, 12 (that is, CYP2C19.5A, CYP2C19.5B, CYP2C19.6, CYP2C19.8, CYP2C19.9, CYP2C19.10, CYP2C19.14, CYP2C19.16, CYP2C19.19, CYP2C19.22, CYP2C19.24 and CYP2C19.25) showed no or markedly low activity compared with the wild-type protein CYP2C19.1B. This comprehensive in vitro assessment provided insights into the specific metabolic activities of CYP2C19 proteins encoded by variant alleles, and this may to be valuable when interpreting the results of in vivo studies.The Pharmacogenomics Journal advance online publication, 8 July 2014; doi:10.1038/tpj.2014.30.