Staphylococcal toxic shock syndrome toxin (TST) inhibited growth of normal human epithelial (Chang) cells in culture, increasing the generation time 28% and 64% at concentrations of 4 X 10(-7)M and 8 X 10(-7)M, respectively. Fluorescence and electron microscopy of the cells treated with TST revealed the location of TST in the coated pits, specialized areas of the cell membrane known to contain high-affinity receptors for other polypeptide ligands. TST was labeled with 125I without detectable damage to the molecule and was shown to bind specifically to epithelial cells. A 100-fold excess of unlabeled TST inhibited binding of 125I-labeled toxin to the cells. Binding data indicated 10(4) receptor sites per cell for TST and a dissociation constant of 4 X 10(-9)M. Specific high-affinity binding of 125I-labeled TST to epithelial cells and the location of receptor sites in coated pits implies a possibility that the toxin is internalized by receptor-mediated endocytosis.