The authors wished to investigate some aspects of the cell-mediated immunity in hemodialyzed subjects undergoing a treatment with recombinant erythropoietin (rHuEPO). The study was carried out through the dosage of the serum levels of neopterin, a substance produced by the macrophages and usable as a marker of their state of activation. The results show a significant reduction of the serum levels of pteridine already at the second month of treatment with rHuEPO (61 +/- 9 vs. 95 +/- 12 nmol/l), thus proving a probable inhibitory action carried out by erythropoietin on certain cells of the immune system.