The structure and heparin binding properties of a family of human cytomegalovirus (HCMV) disulphide-linked glycoprotein complexes designated gC-II were analysed. gC-II complexes contain two groups of glycoproteins designated Group 1 and Group 2. These glycoproteins were separated from each other by short exposure of virions to a reducing agent. This showed that the disulphide bonds between these glycoproteins were on the external surface of the virion. Although these glycoproteins were no longer associated they were not released from the virion, suggesting that they were transmembrane glycoproteins. Approximately 75 to 90% of the gC-II complexes and 18% of the complexes containing the HCMV gB glycoprotein obtained from the virion envelope bound immobilized heparin. When virions were incubated with [3H]heparin, gC-II complexes bound more heparin than gB complexes, by approximately threefold. These data showed that gC-II complexes had a greater heparin-binding capacity. After treatment of virions with a reducing agent the affinity of gC-II glycoproteins for heparin was greatly reduced whereas the affinity of gB glycoproteins was only slightly reduced. Thus, higher order structure was important for heparin binding by gC-II complexes but not by those of gB. Relative to gC-II Group 1 glycoproteins, a greater portion of gC-II Group 2 glycoproteins still bound to heparin after reduction, suggesting that Group 2 glycoproteins may be the important heparin binding component of the gC-II complexes. Both gB and gC-II complexes were eluted from immobilized heparin with soluble heparin or 0.65 M-NaCl suggesting that both formed ionic bonds with heparin. Chondroitin sulphate was not effective at eluting HCMV envelope glycoproteins from immobilized heparin. Thus, the structure of the glucosaminoglycan backbone is important to the binding of HCMV glycoproteins to heparin.