1. Chick embryo in ovo was used to investigate the effects of 1,2-dibromoethane (DBE) on haematopoiesis at a developmental stage where the primitive erythroid cells divide and differentiate in circulation. 2. Early after DBE treatment on embryonic day 3, annexin V/propidium iodide labelling showed acute cell death of erythroid elements, which was subsequently compensated for by the release of immature cells into the circulation. Simultaneously, the comet assay indicated increased DNA damage in DBE-exposed blood cells when compared with controls. 3. After embryonic day 5, there was no indication for ongoing prominent cell death in the DBE-treated group. However, the DNA damage assessed by the comet assay persisted until embryonic day 10 in the peripheral blood cells, and for even longer in cells from thymus and bursa. 4. The kinetics of DNA fragmentation in both erythroid and lymphoid cells implied genotoxic damage by DBE to the stem cells of the definitive elements and transmission of this damage through the successive cell generations. 5. The early chick embryo provides a suitable alternative to mammalian models for investigation of long-term effects of xenobiotics on haematopoiesis.