Outer surface protein A OspA is the major outer surface protein of B. burgdorferi, the causative agent of Lyme disease, and has been advocated as a vaccine candidate. It is recognized late or not at all in the course of human Lyme disease, but has been identified as a major antigenic epitope for the anti-spirochetal immune response in a number of experimental models of B. burgdorferi infection. We injected B.burgdorferi into mice and tested the appearance of immunoreactivity to OspA by Western blotting. Three routes of infection were studied; other variables investigated were inoculum size and isolate of spirochete and strain of mouse. OspA immunoreactivity, as determined by Western blotting, was readily elicited by injection of sonicates under almost any condition. Intraperitoneal or intravenous injection of infectious spirochetes, especially at infective inoculum sizes, or injection of the noninfectious B31 isolate by any route, resulted in OspA immunoreactivity. However, mice from the three strains tested infected intradermally did not develop significant OspA immunoreactivity, but instead developed strong responses to B.burgdorferi proteins of different molecular weights. These data suggest that during infection within the skin after intradermal inoculation, the OspA protein may be altered in some way to make it less immunogenic than when it is presented to the immune system under other circumstances.(ABSTRACT TRUNCATED AT 250 WORDS)