The expression of class II antigens (HLA-DR, DP and DQ) on resting and PHA-activated T cells from renal allograft recipients was studied. A tendency for increase in DR+/DQ+ T cells was noted in azathioprine and cyclosporine-treated recipients undergoing rejection. Low inducible HLA-D expression (especially DP) was associated with CMV infection. Cyclosporine (but not azathioprine) lowered the rate of synthesis of HLA-D by T cells activated in vitro.