The ability of an activated carbon (AC) to adsorb 18 different cytokines with molecular weights ranging from 8kDa to 70kDa and high mobility group box-1 (HMGB1) from inflammatory model plasma at 310K and the mechanisms of adsorption were examined. Porosity analysis using N 2 gas adsorption at 77K showed that the AC had micropores with diameters of 1–2nm and mesopores with diameters of 5–20nm. All 18 cytokines and HMGB1 were adsorbed on the AC; however, the shapes of the adsorption isotherms changed depending on the molecular weight. The adsorption isotherms for molecules of 8–10kDa, 10–20kDa, 20–30kDa, and higher molecular weights were classified as H-2, L-3, S-3, and S-1 types, respectively. These results suggested that the adsorption mechanism for the cytokines and HMGB1 in the mesopores and on the surface of the AC differed as a function of the molecular weight. On the basis of these results, it can be concluded that AC should be efficient for cytokine adsorption.