In some populations of the western black-legged tick, Ixodes pacificus, the prevalence of infection with Lyme disease spirochetes (Borrelia burgdorferi) in nymphal ticks exceeds those in adult ticks by 3-4-fold. Experiments were conducted to determine if the reduced spirochetal prevalence in adult ticks is due to the presence of anti-borrelial antibodies or to another borreliacidal factor in the blood of the western fence lizard, Sceloporus occidentalis, a primary host of subadult I. pacificus, or to loss of spirochetes as nymphal ticks molt to the adult stage. Ten lizards were each exposed to the feeding activities of 10 nymphs having a 78% prevalence of B. burgdorferi infection. Five of the lizards had been hyperimmunized first with 10(8) heat-killed spirochetes and 5 were seronegative to B. burgdorferi. After repletion and the transstadial molt, none of 62 resultant adult ticks from both groups of lizards was found to contain spirochetes. In contrast, 11 of 20 (55%) infected nymphs that had fed on 4 preimmune rabbits passed spirochetes to adult ticks. Taken together, these findings demonstrate that host immunoglobulins and the transstadial molt by themselves are not necessary for eliminating B. burgdorferi from infected nymphal ticks. A novel in vitro assay revealed that nearly all spirochetes placed in plasma or sera from lizards died in less than 1 hr, whereas many spirochetes injected into mouse plasma or sera survived for 72 hr. When spirochetes were put in lizard sera that had been preheated (100 C for 10 min) and allowed to cool, survival was extended to 72 hr. We conclude that the blood of S. occidentalis contains a thermolabile, borreliacidal factor, probably a protein, that destroys spirochetes in the midgut diverticula of feeding I. pacificus nymphs.