The in vivo mutagenesis from ethyl methanesulfonate (EMS) was investigated by the original Pig-a assay, which measures Pig-a mutant cells in red blood cells, and the PIGRET assay, which uses reticulocytes as a part of a collaborative study supported by the Japan Health Sciences Foundation. For the studies with the Pig-a assay, three dose levels of EMS (25, 50 and 100 mg/kg/day) were given orally to 6-week-old male F344 rats for 28 days, and peripheral blood was sampled just before the start of dosing, and after dosing for 1, 2 and 4 weeks with EMS. In the studies with the PIGRET assay, single oral dosing at two dose levels (360 and 720 mg/kg) of EMS was employed and peripheral blood was collected just before dosing, and 1 and 2 weeks after dosing of EMS. As the results, a statistically significant increase in mutant frequency of the Pig-a gene was observed at 2 and 4 weeks, but not at one week after dosing of EMS in the Pig-a assay. In the PIGRET assay, on the other hand, a statistically significant increase in mutant frequency was obtained at one week after the dosing. These results indicate that the PIGRET assay can detect Pig-a mutants much earlier than the original Pig-a assay can by focusing on reticulocytes rather than red blood cells as the target cell population.