Halogenated biphenyl transport by components of rat blood was studied under both in vivo and in vitro conditions. Fractionation of plasma components by gel filtration, ultracentrifugation, and chromatography on a column of fine glass beads indicate that halogenated biphenyls are associated with each major class of plasma proteins but are most concentrated in the lipoproteins. A significant portion of the total halogenated biphenyl in whole blood is also associated with the cellular component. Halogenated biphenyls are readily exchanged between plasma and the cellular component and between lipoproteins and other classes of plasma proteins. Partition of a series of halogenated biphenyls between lipoproteins and other plasma proteins indicated that the relative affinity of a biphenyl for each fraction was proportional to the lipid solubility of the biphenyl involved. Halogenated biphenyls in blood are not thought to be bound to specific sites on blood proteins but rather they are believed to be associated with hydrophobic sites on plasma proteins or the cellular component of blood. The rapid transfer of these compounds to tissues is thought to be by partition to similar sites on cellular proteins.