BACKGROUND Some investigations, incl. ours, provided evidence that recombinant erythropoietin (rHU-EPO) used in the treatment of anaemia in dialyzed patients has an impact also on some immunological parameters. The objective of the present work was to study mechanisms of the effect of rHU-EPO on neutrophil leukocytes. RESULTS The candicidal activity (CI) of polymorphonuclear leukocytes in whole blood (CI-WB), after isolation of cells in the presence of pooled serum (CI-PMN) was assessed and parallel tests were performed in the presence of rHU-EPO in vitro in a concentration of 2 U/ml (CI-WB/E, CI-PMN/E) in anaemic patients before treatment with EHU-EPO (group A, n = 16), after achieving the target haematocrit (0.30-0.35) (group T, n = 14) and in healthy controls (group C, n = 8). The candicidal activity in group A was lower than in controls in whole blood; after removal of uraemic serum it reached normal values. Addition of rHU-EPO in vitro had a marked stimulating effect (CI-WB = 0.16 +/- 0.06, WB/E = 0.22 +/- 0.06*, PMN = 0.23 +/- 0.11*, PMN/E 0.34 +/- 0.1**). After achievement of the desirable Ht the candidacidal activity became normal and isolation of the cells did not change it. In this group a stimulating effect of rHU-EPO in vitro was apparent, contrary to the control group (group T: CI-WB = 0.22 +/- 0.05, WB/E = 0.29 +/- 0.05*, PMN = 0.21 +/- 0.04, PMN/E = 0.27 +/- 0.06*, group C: CI-WB = 0.21 +/- 0.06, WB/E = 0.21 +/- 0.06, PMN = 0.22 +/- 0.06 PMN/E = 0.21 +/- 0.05) (*/**p < 0.01/0.001). CONCLUSIONS The results of the investigation indicate that during treatment with rHU-EPO phagocytic functions in dialyzed patients attain normal levels. This effect may be also due to the direct effect of rHU-EPO on neutrophil leukocytes of dialyzed patients.