We have previously reported that soluble aminated beta-1,3-D-glucan (AG), a potent immunomodulator, specifically inhibited binding and internalization of AG-coated microbeads (GDM) in mouse peritoneal macrophages. The present study was undertaken to determine parameters of AG binding to macrophages. For this purpose, AG was conjugated with tyraminyl cellobiose (TC), which can be radioiodinated. With this method the immunomodulator was labelled with a very high specific radioactivity, allowing sensitive measurements of binding. Maximal binding capacity was 0.33 micrograms [125I]TC-AG/10(6) cells. Binding was inhibited by TC-AG and AG, but not by mannose and mannan, showing that the receptor different from the mannose receptor was involved. Binding was reversible, with an initial association rate of 120 cpm/min, and a much faster initial dissociation rate of 680 cpm/min. Bound [125I]TC-AG was internalized. These findings suggest that both AG and GDM are bound and internalized via the same beta-glucan receptor in mouse peritoneal macrophages.