The in vitro micronucleus (MN) assay is used to evaluate the potential clastogenicity and aneugenicity of a variety of chemicals and agents in vitro. Charles River Laboratories-Skokie previously validated the in vitro MN assay in TK6 cells, according to OECD Test Guideline 487, and we previously performed this assay in CHO-WBL cells in an abbreviated non-GLP screening format. We report here validation of this assay in CHO-WBL cells under TG 487- and GLP-compliant conditions. Both cell types are exposed to test or control articles for four hours with and without metabolic activation (±S9), and for 24 or 27 hours -S9. All CHO-WBL cultures are harvested 24 hours after start of treatment, as are the TK6 cells treated for 27 hours -S9. However, we have found an extended harvest time (44 hours after the start of treatment) provides greater sensitivity for TK6 cells using the short treatment ±S9. Comparisons of our current historical control databases, compiled from at least 10 independent trials, reveals that TK6 cells have lower background MN frequencies for the various treatments than CHO-WBL cells (averaging 0.30 to 0.36 and 0.73 to 0.83 % MN, respectively). Additionally, CHO-WBL cells exhibited lower cytotoxicity to our standard positive controls: mitomycin C (4-hr –S9), cyclophosphamide (4-hr +S9) and vinblastine (24- or 27-hr –S9). While the absolute and fold-increases in MN observed for these positive controls were generally larger in CHO-WBL cells, and they occurred at higher concentrations, positive responses were observed at lower concentrations in TK6 cells, indicating increased sensitivity to clastogens and aneugens. Thus, under these conditions, the more sensitive TK6 cells may be more appropriate to evaluate e-liquids and condensates, which generally have proved to be relatively innocuous, while the more resistant CHO-WBL cells may be more appropriate to compare combusted smoke condensates, which are relatively more cytotoxic.