The immunogenicity and protective efficacy of an attenuated isolate of Borrelia burgdorferi, the agent of Lyme disease, were evaluated. An isogenic pair of isolates of strain HB19 differed in the expression of flagella; neither produced systemic infection or persisted in the skin of mice. In a comparison of intradermally administered live flagella-bearing and flagella-less cells, the flagella-less isolate was equal to or superior to the flagella-bearing wild type in eliciting growth-inhibiting antibodies and preventing infection in mice. In a comparison of live and killed flagella-less cells, immune responses to live cells were superior to those to killed cells, as assessed by ELISA, growth inhibition assay, and infectious challenge. The dose protecting 50% of mice was 10(3.8) and 10(5.2) for live and killed cells, respectively (P < .05).