This Test Guideline describes an in vivo erythrocyte Pig-a gene mutation assay (hereafter called the Pig-a assay) which uses an endogenous mammalian gene, the phosphatidylinositol glycan class A gene (Pig-a), as a reporter of somatic-cell gene mutation. In vivo gene mutation tests, such as the Pig-a assay, are especially relevant for assessing mutagenicity because physiological factors, such as absorption of the test chemical from the site of exposure, distribution of the test chemical throughout the test system via systemic circulation, and in vivo metabolism and DNA repair processes, all contribute to the mutagenic responses. The Pig-a assay can be performed with commonly used strains of rats or mice and the test can be conducted without euthanizing the animals. These properties facilitate integration of the Pig-a assay into many in vivo rodent testing protocols.