Borrelia crocidurae is an etiologic agent of relapsing fever in Africa and is transmitted to humans by the bite of soft ticks of the genus Ornithodoros. The role of the plasminogen (Plg) activation system for the pathogenicity of B. crocidurae was investigated by infection of Plg-deficient (plg(-/-)) and Plg wild-type (plg(+/+)) mice. No differences in spirochetemia were observed between the plg(-/-) and plg(+/+) mice. However, signs indicative of brain invasion, such as neurological symptoms and/or histopathological changes, were more common in plg(+/+) mice. Quantitative immunohistochemical analysis demonstrated infection of spirochetes in kidney interstitium and brain as soon as 2 days postinoculation. Lower numbers of extravascular spirochetes in plg(-/-) mice during the first days of infection suggested a less efficient invasion mechanism in these mice than in the plg(+/+) mice. The invasion of the kidneys in plg(-/-) mice produced no significant inflammation, as seen by quantitative immunohistochemistry of the CD45 common leukocyte marker. However, significant kidney inflammation was observed with infection in the plg(+/+) mice. In brain, inflammation was more severe in plg(+/+) mice than in plg(-/-) mice, and the numbers of CD45(+) cells increased significantly with duration of infection in the plg(+/+) mice. The results show that invasion of brain and kidney occurs as early as 2 days after inoculation. Also, Plg is not required for establishment of spirochetemia by the organism, whereas it is involved in the invasion of organs.