Background <p>The aims of this study were to analyze the relationship between the membrane surface area and cytokine adsorption performance of polymethylmethacrylate (PMMA) hemofilters as well as the distribution of cytokines inside the membrane after contact with a cytokine.</p> Methods <p>Briefly, 1 L of bovine plasma with interleukin (IL)-6 was circulated through four types of PMMA hemofilters with different membrane surface areas (0.3, 0.6, 1.3, and 1.8&#xa0;m²) without dialysis and filtration, and changes in the IL-6 concentration in the plasma were observed. In addition, mini-modules comprising PMMA hollow fibers were circulated in an IL-6 solution for 24&#xa0;h, and the distribution of IL-6 in the PMMA membrane was inferred by detecting the secondary ion image of ¹²C¹⁴N⁻, which reflects nitrogen-containing organic compounds (in this case, IL-6), using a nanoscale secondary ion mass spectrometer (NanoSIMS).</p> Results <p>After 24&#xa0;h of circulation, the IL-6 removal rates for hemofilters with membrane surface areas of 0.3, 0.6, 1.3, and 1.8&#xa0;m² were 74%, 84%, 89%, and 89%, respectively. The times required to reduce the initial concentration by 50% were 486 ± 25&#xa0;min, 175 ± 16&#xa0;min, 104 ± 6.7&#xa0;min, and 88 ± 20&#xa0;min, respectively (<i>p</i> &lt; 0.01). Using a 1.8&#xa0;m² hemofilter, in vitro circulation tests for IL-8, IL-10, and tumor necrosis factor (TNF)-<i>α</i> showed removal rates of 87%, 85%, and 54%, respectively, at 4&#xa0;h. The observed ¹²C¹⁴N⁻ signal indicates a possible distribution of IL-6 across the PMMA hollow-fiber membrane from the blood side to the dialysate side after the IL-6 solution was circulated through the mini-module for 24&#xa0;h.</p> Conclusion <p>PMMA hemofilters rapidly adsorbed IL-6 in an in vitro circulation test, depending on the membrane surface area. IL-8, IL-10, and TNF-<i>α</i> were also removed. IL-6 was inferred to permeate the PMMA membrane and be widely adsorbed throughout the membrane.</p>