Group B streptococci were labeled either by growing the cells in [14C]fructose or by using the surface label 4,4'-[3H]diisothiocyano-1,2-diphenylethane-2,2'-disulfonic acid, which reacts with amino groups. A quantitative assay was developed by using these labeled bacteria to study the adherence of streptococci to canine kidney epithelial cells. The bacteria adhered to kidney cells that had been infected with influenza A virus, but did not adhere to uninfected cells. The binding of 3H-labeled group B streptococci was proportional to the number of bacteria added and showed saturation kinetics. The binding was blocked by the addition of unlabeled group B streptococci but was not affected by addition of streptococci from other groups. It was also blocked by mixing the 3H-labeled streptococci with influenza A virus before adding the bacteria to the kidney cells. When the kidney cells were infected with influenza virus in the presence of either tunicamycin or streptovirudin, these antibiotics inhibited the appearance of viral hemagglutinin in the kidney cells and also prevented the release of mature virus. In these experiments, the adherence of 3h-labeled streptococci was also inhibited. Tunicamycin was shown to block the incorporation of [14C]mannose into lipid-linked oligosaccharides and glycoprotein in both normal and virus-infected kidney cells. These data give strong support to the notion that adherence of streptococci to mammalian cells involves recognition of viral hemagglutinin, a glycoprotein whose synthesis is blocked by certain antibiotics.