Despite cranberry being associated with the prevention of bacterial infections for over a century, our understanding of the bioavailability and mechanisms by which cranberry prevents infection is limited. This study investigates the interactions between cranberry proanthocyanidins (CPAC) and human serum proteins (albumin, alpha-1-acid glycoprotein, and fibrinogen) that may be encountered during CPAC metabolism following ingestion. To better understand how CPAC might interfere with bacterial infection, we also examined the interactions between CPAC and selected bacterial virulence factors; namely, lipopolysaccharide (LPS) and rhamnolipid. The binding of CPAC to the serum proteins, rhamnolipids and LPS from Escherichia coli O111:B4 can be described by Langmuir-type isotherms, allowing the determination of the apparent adsorption affinity constants, with CPAC interacting most strongly with fibrinogen with a binding constant of 2.2 x 10(8) M(-1). These binding interactions will limit the bioavailability of the CPAC at the site of action, an important consideration in designing further clinical trials. Furthermore, CPAC interacts with Pseudomonas aeruginosa 10 LPS, E. coli O111:B4 LPS, and P. aeruginosa rhamnolipids in fundamentally different manners, supporting the theory that cranberry prevents bacterial infections via multiple mechanisms.