An in vitro skin explant model has been used in an attempt to predict the severity of graft-versus-host disease (GVHD) in HLA-identical donor-recipient pairs. The skin explant model involves the use of donor lymphocytes which have been sensitized against recipient lymphocytes in vitro and then co-cultured with the recipient's skin. Thirteen patients were studied in a prospective manner and results from the skin explant model compared with the clinical status of the patient post-transplant showed good correlation (p less than 0.001). Results from T cell-depletion studies indicated a role for both CD4 and CD8 positive cells in GVHD. In conclusion the results confirmed that the skin explant assay is a useful and predictive test of GVHD in humans.