The interaction between polymorphonuclear leukocytes (PMNLs) and Treponema pallidum was studied. Intradermal injection of greater than or equal to 10(6) T. pallidum into rabbits caused a rapid accumulation of PMNLs. Human serum released chemotaxigenic factor (C5a) during incubation in vitro with T. pallidum. Incubation of T. pallidum with human PMNLs in vitro (ratio, 100:1) stimulated chemiluminescence. These responses were dependent upon the presence of both antibody and complement and were greatest when serum from a patient with late secondary syphilis was used as a chemotaxigenic source or for opsonization. Electron microscopic studies documented the rapid uptake of T. pallidum into membrane-bound vacuoles in the human PMNLs in vitro after incubation for as little as 5 min, with leukocyte degranulation and loss of treponemal integrity observed after 4 hr. T. pallidum were found within PMNLs 3 hr after intradermal inoculation of rabbits. These data show that PMNLs are attracted to, and appear to ingest, T. pallidum, but they fail to explain why inoculation of these organisms is not followed by eradication.