Corynebacterium diphtheriae, generally considered an extracellular coloniser, was evaluated for its ability to enter and survive within HEp-2 monolayers by gentamicin protection assay. Intracellular viability of HC01 strain, isolated from endocarditis, was more expressive (2.59%) than observed in 241 (0.21%) and CDC-E8392 (1.93%) strains. Electron microscopy of C. diphtheriae-infected HEp-2 cells revealed intracellular bacteria inside membrane-bound vacuoles. Bacterial internalisation was totally inhibited by 5 microM cytochalasin E and significantly inhibited by 100 microM genistein (P<0.05). Therefore, C. diphtheriae presents the ability to survive within cultured epithelial cells and signalling cascade as well as actin polymerisation are required for entry of diphtheria bacilli into HEp-2 cells.