We have studied the phenotype of mononuclear cells infiltrating target organs of mice with acute graft-versus- host disease after bone marrow transplantation from H-2-identical donors. Infiltrating mononuclear cells with characteristics of large granular lymphocytes (LGL) were frequently found in close association with dead or dying epithelial cells in the skin, liver, and colon. The phenotype of these putative effector cells was Thy-1+, ASGM1 +, Mac-1+, Lyt-1-, Lyt-2-, Ia-, which is characteristic of LGL. Differences in the Thy-1 allele between donor and host were used to demonstrate that these cells were of donor origin. Analysis of cytolytic function in GVHD splenocytes indicated high natural killer activity and low cytolytic T lymphocyte (CTL) activity. These fìndings suggest that an important effector cell in systemic acute graft-versus-host disease is a large granular lymphocyte of donor origin.