We studied peripheral blood CD4-CD8- gamma/delta T cells in recipients of allogeneic marrow grafts, using three-color immunofluorescence and flow cytometry, and investigated changes in their numbers in relation to the administration of hematopoietic growth factors or chronic graft-versus-host disease (GVHD). In the early post-bone marrow transplantation (BMT) period, the relative and absolute numbers of peripheral CD4-CD8- gamma/delta T cells in 22 allogeneic marrow graft recipients in relation to the use of hematopoietic growth factors were studied. During the first 4 weeks, increased numbers of CD4-CD8- gamma/delta T cells were observed in recipients of either recombinant human granulocyte colony-stimulating factor (rhG-CSF) or granulocyte macrophage colony-stimulating factor (GM-CSF). However, 4-8 weeks after BMT, the number of these cells was similar in both groups whether or not growth factors had been administered. At a later stage after BMT (3-12 months), peripheral CD4-CD8- gamma/delta T cells from 43 allogeneic BMT recipients were studied. These cells were markedly decreased in patients with chronic GVHD, and this decrease correlated closely with the clinical signs of chronic GVHD. These results suggest that CD4-CD8- gamma/delta T cells may play an important role in the recovery of neutrophils associated with growth factors during the very early post-BMT period, and in the immunodeficient state of chronic GVHD at a later stage after BMT.