The influence of measles-virus infection on the invasiveness of Shigella flexneri in HEp2-cell cultures was studied. Bacterial invasiveness was significantly enhanced in cell cultures incubated with virus before bacterial inoculation. This effect was a function of time after introduction of virus to the cell cultures and of the concentration of virus. The increase in bacterial invasiveness was observed before production of infectious virus particles and before a cytopathic effect was evident. A similar enhancement of invasiveness was demonstrated when cell cultures were pretreated with UV-inactivated measles virus. Pretreatment of cells with interferon did not influence invasiveness, although it reduced the effect of measles-virus infection.