The tissue distribution of pyran (maleic anhydride-divinyl ether) copolymer was studied after a single ip injection of 14C-labeled pyran (25 mg/kg) to mice. The pyran showed a reticuloendothelial distribution with the liver and spleen containing the highest concentrations which persisted for at least 21 days after drug treatment. Blood levels of 14C-pyran reached a peak 2 hours after injection and were cleared within 6 hours. Attempts to measure uptake of 14C-pyran by peritoneal macrophages were unsuccessful due to an inability to recover macrophages between 3 and 24 hours after ip pyran administration. Since activated macrophages appear to be the primary mechanism by which pyran enhances host resistance to microbial infection and neoplasia, the uptake of 14C-pyran by isolated peritoneal macrophages in vitro was studied. Purified macrophages showed a gradually increasing uptake of 14C-pyran, and a large amount of cell-associated radioactivity was bound to trichloroacetic acid-precipitable material. Several polyanions, including unlabeled pyran, dextran sulfate, and poly(I)-poly(C), competed for acid-precipitable receptor molecules. The superior antitumor effects of pyran as compared to other polyanions may result from the continuous presence of the synthetic polymer in the host. Possible mechanisms of immunopotentiation by pyran are discussed.