Recombinant human granulocyte colony-stimulating factor (rhG-CSF) is frequently used to mobilize CD34+ cells in healthy donors and patient with malignant diseases prior to peripheral blood stem cell (PBSC) harvest. To analyze the effects of rhG-CSF on morphology and genotype of white blood cells, a novel multiparametric cell scanning system that combines morphologic, immune and genotypic analyses of the same cells was used. We report here that tetraploid myeloid cells are present in the peripheral blood of donors treated with rhG-CSF. The tetraploidy was detected in up to 0.6% of differentiated myeloid cells and all observed CD34+ cells were diploid. Thus, short treatment with rhG-CSF of PBSC donors induces numerfical chromosomal alterations in a small subset of mature myeloid cells.