Processes influencing organic carbon distribution and composition can control the speciation of organic contaminants such as polychlorinated biphenyls (PCBs) and ultimately determine their residence time in aquatic environments. Protozoan grazers are active in the remineralization and recycling of organic material both in the water column and at the sediment-water interface. Thus, they influence the quality and quantity of potential PCB binding substrates in the suspended and dissolved phases of aqueous systems. In this study, common headspace systems were used to compare the chlorobiphenyl-binding affinity of dissolved organic carbon (DOC) in protozoan and bacterial culture filtrates (<0.2 microm) relative to DOC in a seawater control. Culture filtrates from three marine protozoan species were compared-Uronema sp., Cafeteria sp., and Paraphysomonas imperforata. Each protozoan species was fed the same bacterial prey, Halomonas halodurans, which was also used as a bacterial control. Affinities of culture DOC for [14C]3,3',4,4'-tetrachlorobiphenyl (IUPAC 77) were normalized to DOC and surfactant concentrations. Values of DOC equilibrium partition coefficients (K(DOC)) ranged from 10(4.6) in seawater (Vineyard Sound, MA) to 10(5.4) and 10(5.5) in protist cultures, indicating that grazer-modified DOC was a better sorbent for PCBs than DOC in bacterial or seawater controls.