Lysine influx (2 microM) into activated human B and T lymphocytes through transport systems y+ and y+L was measured. In activated T cells very substantial activation of system y+ was detected; system y+L was also activated in these cells but with a slower time course and to a smaller extent. No stimulation of either system was found in activated B cells. The time course of activation of system y+ precisely matched the expression of the T cell early antigen gene described by MacLeod et al, 1990. The functional significance of these observations with respect to L-arginine transport and nitric oxide synthesis in activated T lymphocytes is discussed.