In 18 cases of drug-induced acute leukemia, electron microscopy brought out a number of anomalies found in all hematopoietic cell lines. They are described here for the erythroblastic line. Some of these anomalies occur in the cytoplasm (like siderosomes, iron-containing mitochondria, glycogen spots, various dense inclusions, and nuclear-cytoplasmic asynchronism; others occur in the nucleus (such as plurinucleated cells, fragility of nuclear membrane, and nuclear clefts and blebs). Nuclear clefts and blebs may also be found in granulocytic and megakaryocytic cell lines. Their structural aspect suggests a relationship with chromosomal abnormalities, particularly with synaptonemal complexes. Unusual karyotypes, observed in two cases, give weight to such a hypothesis. The cell injury caused by alkylating agents, including DNA lesion, is most likely closely related to this particular pattern of drug-induced leukemias.