A flow cytometric procedure for scoring micronuclei in mouse peripheral blood erythrocytes, especially reticulocytes, is described. The methods reported herein were developed in an effort to simplify the techniques and to reduce the equipment requirements associated with automated micronucleus analyses. With this procedure, fluorescein-conjugated monoclonal antibodies which bind to the CD71-defined antigen (the transferrin receptor) are used to label reticulocytes. The nucleic acid dye propidium iodide is used to identify cells with micronuclei. Given 488 nm excitation, four populations of erythrocytes are clearly resolved: normochromatic erythrocytes with and without micronuclei, and reticulocytes with and without micronuclei. Since the method is capable of simultaneously providing the incidence of micronuclei in both mature and immature erythrocyte populations, it is compatible with either chronic or acute treatment regimens. To demonstrate cell handling and flow cytometric procedures for quantitatively analyzing peripheral blood micronuclei, an experiment with the model clastogen methyl methanesulfonate is described. Additionally, a reconstruction experiment was performed whereby three mouse blood samples were spiked with successively greater volumes of blood from a clastogen-treated animal so each preparation differed slightly, but definitely, in micronucleus content. Each sample was scored six times by conventional microscopy and by flow cytometry so that the two methods could be directly compared. Collectively, the results from the methyl methanesulfonate experiment and the reconstruction study demonstrate the accuracy and reliability of the flow cytometric method. Furthermore, advantages associated with objective, high throughout scoring methodology are clearly indicated.