Two endpoints of genetic toxicity, mutation at the X-linked Pig-a gene and chromosomal damage in the form of micronucleated reticulocytes (MN-RETs), were evaluated in blood samples obtained from 28-day repeat-dosing studies typical of those employed in toxicity evaluations. Male Wistar Han rats were treated at 24-h intervals on days 1 through 28 with one of five prototypical genotoxicants: N-ethyl-N-nitrosourea, 7,12-dimethyl-12-benz[a]anthracene, 4-nitroquinoline-1-oxide (4NQO), benzo(a)pyrene, and N-methyl-N-nitrosourea. Flow cytometric scoring of CD59-negative erythrocytes (indicative of glycosylphosphatidylinositol anchor deficiency and hence Pig-a mutation) was performed using blood specimens obtained on days -1, 15, 29, and 56. Blood specimens collected on days 4 and 29 were evaluated for MN-RET frequency using flow cytometry-based MicroFlow Kits. With the exception of 4NQO, each chemical induced significant increases in the frequency of MN-RETs on days 4 and 29. All five agents increased the frequency of mutant phenotype (CD59 negative) reticulocytes (RETs) and erythrocytes. Mutation responses in RETs occurred earlier than in erythrocytes and tended to peak, or nearly peak, at day 29. In contrast, the mutant phenotype erythrocyte responses were modest on day 29 and required additional time to reach their maximal value. The observed kinetics were expected based on the known turnover of RETs and erythrocytes. The data show that RETs can serve as an appropriate indicator cell population for 28-day studies. Collectively, these data suggest that blood-based genotoxicity endpoints can be effectively incorporated into routine toxicology studies, a strategy that would reduce animal usage while providing valuable genetic toxicity information within the context of other toxicological endpoints.