The acquisition of lactoferrin and transferrin by live Treponema pallidum organisms was examined. Saturation binding kinetics were obtained for virulent treponemes with increasing amounts of radioiodinated lactoferrin but not with transferrin. Furthermore, lactoferrin bound up to 100 times more effectively than transferrin. Only unlabelled lactoferrin stoichiometrically completed with iodinated lactoferrin binding. Time course kinetics showed maximum lactoferrin acquisition within the first five minutes at 34 degrees C. Optimum iron accumulation, however, was achieved by T pallidum in 30 minutes at 34 degrees C, and amounts of iron were six times greater than the equivalent amount of lactoferrin bound. Interestingly, iron uptake was also detected in the presence of transferrin, despite the minimal amounts of transferrin acquired by live treponemes. These observations reinforce the possibility that the associations of T pallidum with host molecules, such as plasma proteins, are essential for survival of the parasite within host environments.