The course of L. monocytogenes infection was followed in mice treated with pentoxifylline (POF), a known inhibitor of endogenous tumor necrosis factor (TNF) formation. Administration of POF caused a delay in L. monocytogenes elimination which was probably related to a reduction the listericidal activity of macrophages and to an attenuation of delayed type hypersensitivity (DTH) to Listeria antigens. In spite of this, some POF-treated mice were protected from lethal effects of virulent L. monocytogenes bacteria.