A murine model of minor histocompatibility antigen (miHCag)–mismatched bone marrow transplantation (BMT) was used to study the development of immunoregulatory cells in the posttransplantation period and their possible involvement in the dissociated graft-versus-host (GVH) and graft-versus-leukemia (GVL) reactivity of posttransplantation donor lymphocyte infusions (DLIs). DLI, applied immediately after BMT, induced GVH disease (GVHD), but when DLI was delayed for 3 weeks, GVHD was avoided while a distinct GVL response was allowed to develop. A population of Mac1⁺Ly6-G⁺Ly6-C⁺ immature myeloid cells, found in small numbers in normal mice, strongly expanded in spleens of chimeras, reaching a maximum level at week 3 and returning to base level by week 12. Upon isolation, these cells exhibited interferon-γ (IFN-γ)–dependent, nitric oxide (NO)–mediated suppressor activity toward in vitro alloresponses, suggesting that, after in vivo DLI, they are activated by IFN-γ to produce NO and suppress GVH reactivity. Because not only alloactivated T-cell proliferation but also leukemia cell growth was found susceptible to inhibition by exogenous NO, in vivo activation of these cells after DLI may explain the occurrence of a GVL effect despite suppression of GVHD. This suggested sequence of events was supported by the finding that the ex vivo antihost proliferative response of spleen cells, recovered shortly after in vivo DLI, was characterized by strong mRNA production of the monokines interleukin-1 (IL-1), IL-6, and tumor necrosis factor-α (TNF-α) and of inducible nitric oxide synthase (iNOS). Our data suggest that transiently expanding Mac1⁺Ly6-G⁺Ly6-C⁺ immature myeloid cells (probably as a result of extramedullary myelopoiesis) may play a role in controlling GVH while promoting GVL reactivity of DLI after allogeneic BMT.