Background/Aims: Circulating CD14+CD16+ monocytes, a potent phagocytosing and antigen-presenting monocyte population, have been reported to be expanded in patients on hemodialysis (HD). In this study, changes in the population of CD14+CD16+ monocytes were analyzed during a single session of HD therapy, and the influence of dialyzer membrane materials on these monocytes was investigated. Methods: Nine patients were hemodialyzed using regenerated cellulose (RC) membranes and thereafter polysulfone (PS) membranes. Peripheral blood cells were taken from these subjects, and these cells were stained with anti-CD14 and anti-CD16 antibodies. The percentages of CD14- and CD16-expressing monocytes were analyzed by two-color flow cytometric analysis. Moreover, the serum soluble CD14 (sCD14) levels were measured with an ELISA kit. Results: It was found that CD14+CD16+ monocytes before HD were significantly increased in patients on HD as compared to healthy controls. In the RC group, CD14+CD16+ monocytes were decreased at both 30 and 240 min after the initiation of HD. The reduction rate of CD14+CD16+ monocytes in the RC group was higher than that in the PS group. There was no significant difference in sCD14 levels between the two groups. Conclu sion: Monocytes are activated in patients on HD. Furthermore, the population of CD14+CD16+ monocytes was stimulated to a greater extent during HD in the RC group than in the PS group. The significant reduction in CD14+CD16+ monocytes by RC membranes indicated that the level of CD14+CD16+ monocytes is a sensitive marker for the biocompatibility of HD membranes.