The induction of micronucleated reticulocytes in the bone marrow is a sensitive indicator of chromoso-mal damage. Therefore, the micronucleus assay in rodents is widely used in genotoxicity and carcino-genicity testing. A test system based on cultured human primary cells could potentially provide better prediction compared to animal tests, increasing patient safety while also implementing the 3Rs principle, i.e. replace, reduce and refine. Hereby, we describe the development of an in vitro micronucleus assay based on animal-free ex vivo culture of human red blood cells from hematopoietic stem cells. To validate the method, five clastogens with direct action, three clastogens requiring metabolic activation, four aneu-genic and three non-genotoxic compounds have been tested. Also, different metabolic systems have been applied. Flow cytometry was used for detection and enumeration of micronuclei. Altogether, the results were in agreement with the published data and indicated that a sensitive and cost effective in vitro assay to assess genotoxicity with a potential to high-throughput screening has been developed.