Histidine-rich glycoprotein (HRG), a plasma protein that binds heparin and divalent cations, has been implicated in immune regulation through its ability to modulate complement function, macrophage Fc receptor expression and phagocytosis, and its ability to inhibit the proliferation of human peripheral blood T cells in vitro. In the present work we used fluorescence flow cytometry to study the binding of human HRG to the human T-cell lines Jurkat and MT4, and to the murine antigen-specific T-cell clone D10, and to study the effect of divalent cations zinc and copper on this binding. Our results show that HRG binds strongly to these cell lines at 4 degrees, and that the binding is markedly potentiated by physiological concentrations of zinc (20 microM), and to a lesser extent by copper (10 microM). In contrast to previous studies, HRG binding was largely inhibited by 50 micrograms/ml heparin, both in the absence and in the presence of zinc, suggesting that HRG interacts primarily through glycosaminoglycans on the T-cell surface. Studies using confocal fluorescence microscopy indicated that following incubation of MT4 cells with HRG in the presence of zinc at 4 degrees, the HRG was localized exclusively at the plasma membrane, but was actively internalized after incubation at 37 degrees. Interestingly, HRG interfered with the ability of D10 cells to adhere to tissue culture plastic, as well as to laminin-, collagen- or fibronectin-coated culture dishes. This effect was markedly potentiated by 20 microM zinc, and was partially reversed by heparin. The results suggest that zinc markedly potentiates the binding of HRG to T cells, and that HRG and zinc may play an important role in regulating the adhesion of T cells to other cells and the extracellular matrix.