Mice deficient or deprived of thymus-derived lymphocytes eliminated blood-borne Borrelia turicatae with efficiency comparable to that observed in normal littermates. When challenged with 10(5) borreliae, nude mice had mean (+/- standard deviation) primary spirochetemias lasting 3.1 +/- 0.2 days and mean (+/- standard deviation) peak bacterial counts of 3.0 X 10(7) +/- 0.5 X 10(7) cells per ml of blood; in comparison, heterozygous littermates and normal mice had respective primary spirochetemias lasting 3.4 +/- 0.6 and 3.2 +/- 0.2 days and respective peak counts of 8.0 X 10(7) +/- 1.5 X 10(7) and 5.5 X 10(7) +/- 0.9 X 10(7) bacterial cells per ml of blood. No increased responsiveness to concanavalin A was observed in infected nude mice, indicating the sustained lack of maturate T cells in these animals. Thymectomized and steroid-treated mice were also found to eliminate circulating borreliae with efficiency comparable to that observed in control animals. Irradiation of mice abrogated responsiveness to borreliae, but reconstitution with T-cell-depleted splenocytes restored antibody production. It is proposed that elimination of B. turicatae is mediated by a T-cell-independent immune response mechanism.