Immunotherapy with intravesical bacille Calmette-Guérin (BCG) is the treatment of choice against superficial bladder cancer recurrences. However, this therapy is associated with side effects that are considered to be the result of inflammatory cytokines. Since pentoxifylline is known to interfere with the production of cytokines, this drug was tested in vitro with regard to a later clinical application in BCG-treated patients. The cytokine release and the cytotoxicity of interleukin-2 or BCG-stimulated mononuclear cells were analyzed, and the growth of BCG under the influence of pentoxifylline was assayed. The results showed an inhibition of cytokine release of stimulated mononuclear cells. The cytotoxicity of BCG-stimulated mononuclear cells but not of lymphokine-stimulated mononuclear cells against bladder carcinoma cells was significantly inhibited. Restimulation with fresh BCG restored cytotoxicity. Direct coincubation of BCG and pentoxifylline resulted in a reduction of mycobacterial metabolism. From these data, we conclude that the use of pentoxifylline to reduce BCG-related side effects should be tested further in a clinical study.