The CD3+CD5- subpopulation of T cells has been shown to be increased in patients following allogeneic bone marrow transplantation, and a statistical association has been found with graft-versus-host disease (GVHD). We studied this population in renal transplant recipients. There was no correlation with rejection episodes but we found an increase in this CD3+CD5- population in patients on cyclosporin, and we suggest that these cells may be involved in the mechanism of action of this drug. In patients on azathioprine the absolute number of CD3+CD5- lymphocytes is reduced, along with other lymphoid subpopulations.