Summary:It has been suggested that the immunological properties of cytokine primed PBSC may reflect the presence of altered levels of cellular components. In this study the changes induced in blood dendritic cell (DC) subsets following G-CSF mobilisation are analysed. Analysis of normal donors (n = 64) demonstrated considerable individual variation in the absolute numbers (؋10 6 /l) of resting blood CD11c ؊ DC (1.2-26.2) and CD11c + DC (0.9-34.7) as well as in the CD11c ؊ /CD11c + DC ratio (0.29-4.13). G-CSF therapy increased CD11c ؊ DC numbers to above the normal range in all normal donors analysed (n = 6) and the CD11c ؊ /CD11c + ratio was also increased to Ͼ2.0 in all donors. Patients undergoing autologous PBSCT showed a heterogeneous response to mobilisation and although total DC and CD11c ؊ DC numbers were increased in the majority (8/14), they remained within the normal range post mobilisation. The CD11c ؊ /CD11c + ratio decreased in 5/15 patients and only three patients had ratios Ͼ2.0 post mobilisation. Post G-CSF the DC from all normal donors and 13/14 patients had an immature phenotype. These results demonstrate that G-CSF mobilisation induces relatively consistent changes in the number and ratio of DC subsets in normal donors, but considerable variation is seen in the response of patients undergoing mobilisation for autologous PBSCT.