To investigate the direct impact of membrane plasmapheresis on the distribution of lymphocyte subsets, paired blood specimens from 18 healthy volunteers were studied before and immediately after a single session of double-filtration plasmapheresis (DFP). After a single session of DFP treatment, the number of helper T cells had increased by 2.34% (P = .0142), whereas the suppressor T cells decreased by 2.22% (P = .0095), with consequent increases in the T-helper/T-suppressor (Th/Ts) ratio (P < .0001). The number of B cells was also significantly increased (P = .0012) after DFP treatment; however, total T and natural killer cells did not differ after treatment. Older volunteers ( > 40 years) had significantly higher percentages of B (P = .0002) and helper T (P = .0432) cells after treatment. In contrast, younger subjects had a lower percentage of suppressor T cells (P = .0174). However, the Th/Ts ratio increased significantly irrespective of age group (P = .0016) and sex (P = .0016). A single session of membrane plasmapheresis seemed to activate the cellular immune system in our sample of 18 healthy volunteers, increasing the number of B cells and the Th/Ts ratio.