We explored the mechanism for the increase of blood IL-6 level after anti-IL-6 receptor (IL-6R) antibody injection. First, we examined whether anti-IL-6R antibody stimulates IL-6 production. Single injection of tocilizumab (anti-IL-6R antibody) in monkeys with collagen-induced arthritis (CIA) caused a marked increase in blood IL-6 and IL-6R levels, but did not increase IL-6 mRNA and IL-6R mRNA expression in liver, spleen, lymph nodes, synovium or whole blood 1, 3 and 7 days later. This suggests that tocilizumab did not induce IL-6 and IL-6R production. Second, we investigated whether anti-IL-6R antibody releases IL-6 from IL-6 complexes in the blood. When plasma from CIA monkeys was incubated with tocilizumab, the IL-6 concentration was not affected. Finally, we studied whether anti-IL-6R antibody affects the clearance of IL-6 from the blood. When MR16-1 (anti-mouse IL-6R antibody) was injected into IL-6-deficient mice continuously infused with human IL-6, blood human IL-6 levels significantly increased. These results suggest that the elevation of blood IL-6 after the administration of anti-IL-6R antibody is the result of inhibition of the clearance of IL-6 due to IL-6R blockade, and that it is not the result of induction of IL-6 production or release of IL-6 from complexes.