Lymphocytes obtained from uraemic patients maintained on intermittent haemodialysis had a normal ability to respond to and stimulate allogeneic lymphocytes obtained from normal subjects in the mixed lymphocyte culture (MLC) reaction. The response of these uraemic lymphocytes to PHA and pokeweed mitogen (PWN) was also normal. Uraemic plasma from eight out of twenty-six patients studied, however, possessed blocking factor activity which suppressed the MLC reactivity of normal random donors and also the mitogenic response of allogeneic lymphocytes but not of autologous uraemic lymphocytes. The blocking factor activity was attributed to a non-dialysable factor present in the plasma of the patients investigated.