Background PBSC transplant provides 10 times more T cells than BMT. However, the incidence and severity of acute GvHD is similar among recipients of both types of transplants. Studies in mouse models suggest that the similar clinical outcome in BMT and PBSCT is due to differences in the lymphokine profiles. Methods PBMC, PBMC from G-CSF mobilized donors (G-PBMC) and BM mononuclear cells (BM-MC) were analyzed by flow cytometry and ELISA to detect γ-IFN and IL-4 production. Hematoxylin and eosin staining was used to identify morphology and annexin/propidium-iodide was used for apoptosis assays. Results We show decreased production ofy-interferon (85%) and IL-4 (60%) in G-PBMC when compared with either PBMC or BM-MC T cells on ex vivo assays. Surprisingly, 85% of fresh G-PBMC is composed of low-density granulocytes (LDG), which undergo apoptosis after 48 h in culture. At this same time, γ-IFN production from G-PBMC T cell was reverted. In vitro, G-CSF converts granulocytes into LDGs, able to inhibit T-cell function by H2O2 production, and not through immune-deviation towards a Th2-type phenotype. Discussion We show that the estimated numbers ofThl and Th2 cells infused in BMT and PBSCT do not differ significantly. These findings are discussed with reference to the relatively low incidence of acute GvHD in PBSCT shown in the literature. We suggest that these results might depend on the high number of granulocytes and progenitors infused. The potential use of granulocytes as immunosu-pressive short-term therapy is now being investigated by our group using a mouse experimental model.