Flow cytometric enumeration of micronuclei in the in vivo micronucleus (MN) test in rodents is an accepted, well validated test in use all over the world. However, its validation for use in human samples is still incomplete due to the very low number of immature reticulocytes carrying the transferrin receptor CD71 in human blood and due to the great variability that they show in response to medical treatments and exposure to environmental chemicals. Our study is a proposal of a flow cytometric method for enumerating micronucleated reticulocytes in human peripheral blood, based on the classical identification of cytoplasmic content of RNA using thiazole orange. It is demonstrated that the use of this dye in combination with Hoechst 33342 to identify DNA allows a robust calculation of MN frequency in microliter quantities of blood samples from children and young adults.