E+-cells were studied in 16 patients on continuous ambulatory peritoneal dialysis (CAPD) to evaluate the impairment of cell-mediated immunity. E-rosette forming cells (E-RFC) were below the normal range at the beginning of treatment in 10/16 patients, after which their number increased and reached normal levels in the majority of patients in three to six months. In this phase of therapy, the same result was obtained with OKT11 monoclonal antibody, while OKT+4/OKT+8 ratio was in the normal range. Normal human lymphocytes, pre-incubated with uraemic peritoneal fluid, showed a significant reduction of E-RFC. Maximum inhibition was observed with the less than 500 daltons fraction of peritoneal fluid. Extraction with chloroform almost completely abolished inhibitory activity, suggesting that the toxic substance(s) has the characteristic of a polar lipid. Immunodeficiency in CAPD patients seems therefore partly restored by the removal through the peritoneum of inhibitors capable of blocking sheep-cell receptors.