Tritrichomonas foetus is an important protozoan parasite of the bovine vaginal tract. It is known that this parasite can use lactoferrin as a source of iron. It has been demonstrated by others that lactoferrin-colloidal gold conjugates bind to and are internalized into the parasite. However, the biochemical mechanisms of binding are not well understood. Since lactoferrin is a very basic protein, it is unclear whether binding to the parasite is due to electrostatic charge interactions and/or requires specific receptors. We have examined the binding of radiolabeled Tritrichomonas foetus proteins to lactoferrin in the presence of various charged and uncharged polypeptides. Radiolabeled parasites were lysed in detergent and the lysates incubated with lactoferrin-Sepharose. After extensive washes, the specifically bound proteins were eluted off the lactoferrin-Sepharose using a low pH buffer followed by neutralization. The radiolabeled proteins were then rebound back to lactoferrin-Sepharose in the presence of lactoferrin or poly-amino acids. The rebinding to lactoferrin was strongly inhibited by lactoferrin. Charged poly-amino acids such as poly-lysine and poly-glutamate did inhibit rebinding in a dose-dependent manner but this inhibition was no greater than 30-40% of that observed using lactoferrin as an inhibitor. In contrast, poly-glycine had no inhibitory effect on binding at any concentration. Taken together, the data suggest that the binding adhesins on the surface of the parasite, specifically bind to lactoferrin