The present invention relates a method for the enumeration of in vivo gene mutation. The method utilizes differential staining of GPI-anchor deficient erythrocyte populations to distinguish between wild-type and pig-a gene mutants. Quantitative analyses can be conducted on erythrocytes and/or reticulocytes, and is based upon fluorescent emission and light scatter following exposure to an excitatory light source. Counting of mutant erythrocytes or reticulcoytes relative to the number of total erythrocytes or reticulocytes can be used to assess the DNA-damaging potential of an exogenous chemical agent, the DNA-damaging potential of an exogenous physical agent, the effects of an exogenous agent which can modify endogenously-induced DNA damage, and the effects of an exogenous agent which can modify exogenously-induced DNA damage. Kits for practicing the invention are also disclosed.