Adenosine has anti-inflammatory activity. Adenosine deaminase (EC 3.5.4.4) metabolizes extracellular adenosine, resulting in an exacerbation of inflammation. Consequently, it was hypothesized that adenosine deaminase inhibitors produce anti-inflammatory activity by increasing extracellular adenosine concentration. This group recently developed a non-nucleoside adenosine deaminase inhibitor, FR234938, by using rational structure-based drug design. FR234938 inhibits recombinant human adenosine deaminase enzyme competitively. FR234938 inhibits interleukin (IL)-6-dependent immunoglobulin (Ig) M production by SKW6.4 cells, in the presence of adenosine. Inhibitory effect of FR234938/adenosine combination is blocked by an A2a adenosine receptor antagonist. FR234938 also inhibits anti-type II collagen delayed type hypersensitivity (DTH) in a dose-dependent manner, both in the presence and absence of recombinant human adenosine deaminase. Moreover, FR234938 inhibits tumor necrosis factor (TNF)-alpha and IL-10 production in a lipopolysaccharide (LPS)-induced cytokine production model in mice. These results indicate that FR234938 has potential anti-inflammatory activity. Non-nucleoside adenosine deaminase inhibitor FR234938 has good potential as a new type of anti-rheumatic and anti-inflammatory drug, by modulating host-defense concentrations of adenosine.