Although recent evidence suggests that fibronectin may be involved in the attachment of treponemes to mammalian cells, its possible role in promoting phagocytosis of Treponema pallidum has not been investigated. In the present study, we examined the antibody-independent interactions of fibronectin, C1q, and human polymorphonuclear leukocytes with T. pallidum. Binding of [125I]fibronectin was specific and saturable with an affinity constant of approximately 2 X 10(7) M-1. The number of binding sites per treponeme at 37 degrees C, irrespective of the mammalian source of fibronectin, was between 2,500 and 7,500, with a mean of approximately 4,700. Binding of [125I]C1q to T. pallidum, in the absence of antibodies to the organism, also was saturable and specific. Pretreatment of treponemes with C1q enhanced binding of soluble [125I]fibronectin two- to threefold and also increased attachment of 125I-surface-labeled treponemes to fibronectin-coated surfaces. Treatment of 125I-labeled T. pallidum with fibronectin alone, or together with C1q, however, did not enhance surface phagocytosis by neutrophils.