We have produced, by using a sonicate of Borrelia burgdorferi, a monoclonal antibody (MAb), NYSP39H, that is specific for the P39 protein band. This MAb reacted with 13 isolates of B. burgdorferi but not with eight different spirochetes (four borrelias, two leptospiras, and two treponemas). Surface labeling of B. burgdorferi with biotin and subsequent treatment with Nonidet P-40 showed that P39 was not biotinylated but was extracted with Nonidet P-40, indicating that it is present within the outer membrane, but not on the surface of the spirochete. Immunoelectron microscopy revealed the immunogold probe primarily at the cytoplasmic membrane region of the spirochete. The MAb detected B. burgdorferi in the indirect fluorescent-antibody test only when the spirochetes from a culture or in a tick homogenate were fixed with polylysine and not with acetone. NYSP39H appears to be an appropriate probe for use in the specific detection of B. burgdorferi.