Although reactivity in nontreponemal tests develops in patients with untreated syphilis, no immunologic function has been ascribed to these antibodies. This study demonstrates that rabbit antibodies induced by immunization with VDRL antigen and VDRL antibodies affinity-purified from syphilitic rabbit serum enhance phagocytosis of Treponema pallidum. The proportion of macrophages ingesting treponemes in the presence of these antisera was 45% +/- 5% and 27% +/- 4%, respectively, versus 14% +/- 3% for normal serum (P < .001 and P < .01). Both IgG and IgM fractions contained opsonic activity. Absorption of VDRL antibodies from syphilitic serum diminished but did not eliminate opsonization, suggesting at least two classes of target molecules. Despite opsonic capacity, VDRL antibodies fail to facilitate macrophage-mediated killing of T. pallidum. Nevertheless, VDRL-immunized rabbits are partially protected against T. pallidum infection, developing fewer lesions (delayed and smaller) than do unimmunized controls. These results suggest a heretofore unrecognized functional role for VDRL antibodies in syphilis infection.