The effect of salivary gland extract (SGE) and saliva from the tick Ixodes ricinus (L.) on the interaction of Borrelia afzelii spirochetes with mouse macrophages as well as on the borreli-acidal effect of calf serum was studied. SGE reduced both the number of phagocytosing cells and phagocytosed bacteria. An inhibitory effect of SGE on the killing of spirochetes by the alternative pathway of complement activation also was observed. Both SGE and saliva down-regulated production of proinflammatory cytokine tumor necrosis factor-alpha by macrophages stimulated with interferon-gamma and live B. afzelii spirochetes. The production of another macrophage cytokine, interleukin-6, remained unchanged. SGE and saliva exerted a different effect on the production of nitric oxide by stimulated macrophages. Whereas SGE up-regulated NO production, saliva decreased it. The significance of immunosuppressive effects of tick saliva for the transmission of Borrelia spirochetes is discussed.