The influence of infection of HEp-2 cells with Coxsackie B 1 virus on the invasiveness of Salmonella typhimurium has been studied. The bacterial invasiveness was significantly increased in the cell cultures incubated with virus before bacterial inoculation. This effect was a function of time after introduction of virus into the cell cultures and the concentration of virus. The increase in bacterial invasiveness was observed before production of infectious virus particles and before development of cytopathogenic effect was evident. Two strains of non-invasive Escherichia coli did not show invasiveness after the virus treatment of the cells. The results indicate a specific mechanism for the interaction.