Supernatant culture fluids from dengue virus type 4 (DEN-4)-infected cultures of monkey kidney Vero cells and Aedes albopictus C6/36 cells contained the virion structural proteins; secreted NS1 was found only in supernatants from infected Vero cells. Using supernatant culture fluids from [35S]methionine-labelled, virus-infected Vero and C6/36 cells, binding of radiolabelled viral proteins was examined with various cell lines varying in susceptibility to DEN-4 infection. Binding of viral E protein was observed with the highly infectible Vero and LLC-MK2 cell lines, whereas a very small degree of binding was seen with four other cell lines (mouse fibroblast L929, bovine kidney MDBK, human hepatoma Hep G2 and primary human endothelial cells) which are less susceptible to DEN-4 infection. The results suggest that cell susceptibility to DEN-4 may be determined largely at the stage of virus binding, i.e. by the presence of a cell receptor capable of binding viral E protein.