In this work we characterize the interaction of cranberry (Vaccinium macrocarpon) proanthocyanidins (PAC) with bovine serum albumin (BSA) and hen egg-white lysozyme (HEL) and determine the effects of these complexes on macrophage activation and antigen presentation. We isolated PAC from cranberry and complexed the isolated PAC with BSA and HEL. The properties of the PAC-protein complexes were studied by matrix assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS), gel electrophoresis and zeta-potential. The effects of PAC-BSA complexes on macrophage activation were studied in RAW 264.7 macrophage like cells after treatment with lipopolysaccharide (LPS). Fluorescent microscopy was used to study endocytosis of PAC-BSA complexes. The effects of PAC complexes on macrophage antigen presentation was studied in an in vitro model of HEL antigen presentation by mouse peritoneal mononuclear cells to a T-cell hybridoma. Mass spectra of PAC complexes with BSA and HEL differed from spectra of the proteins alone by the presence of broad shoulders on the singly and doubly charged protein peaks. Complexation with PAC altered the electrophoretic mobility shift assay in native agarose gel and the electrophoretic mobility (ζ-potential) values. These results indicate that the PAC-protein complexes are stable and alter protein structure without precipitating the protein. Fluorescent microscopy showed that RAW 264.7 macrophages endocytosed BSA and PAC-BSA complexes in discrete vesicles that surrounded the nucleus. Macrophages treated with increasing amounts of PAC-BSA complexes had significantly reduced COX-2 and iNOS expression in response to treatment with lipopolysaccharide (LPS) in comparison to controls. PAC-HEL complexes modulated antigen uptake, processing and presentation in murine peritoneal macrophages. After 4 h of pre-incubation, only trace amounts of IL-2 were detected in the co-cultures treated with HEL alone, whereas a PAC-HEL complex had already reached maximum IL-2 expression. Cranberry PAC may increase rate of endocytose of HEL and subsequent expression of IL-2 by the T-cell hybridomas. These results suggest that PAC-protein complexes modulate aspects of innate and acquired immune responses in macrophages.