An indirect immunofluorescence technique for the determination of antibodies against ixodid tick spirochetes is described. Differences in the reactivity between Ixodes ricinus spirochete and Ixodes dammini spirochete antigens were not observed. Cross-reacting antibodies against Treponema pallidum and Treponema phagedenis can be eliminated by quantitative absorption with T. phagedenis. Cross-reactions with leptospira were not observed by immunofluorescence. In the IgM test, false negative reactions caused by high-titered specific IgG antibodies or false positive reactions caused by rheumatoid factor occur. This can be avoided by testing the IgM fraction (19S-IgM-test) or using sera previously treated with anti-IgG serum. Significantly elevated antibody titers against ixodid tick spirochetes were observed in 45% of 44 cases with erythema migrans disease, in 72% of 29 cases of lymphocytic meningoradiculitis, in all of nine patients with acrodermatitis chronica atrophicans and in all of four investigated patients with lymphocytoma (lymphadenosis benigna cutis).