A strain of Shigella flexneri type 2a was found to multiply intracellularly in cultures of the diploid human cell strain FS-1 and in secondary rabbit kidney cells. When inoculated cultures were stained and observed under the light microscope, it appeared that the cytoplasm of infected cells became gradually filled with bacteria. Various preparations of human and rabbit interferon were found to suppress the intracellular bacterial growth in homologous cells. Polyinosinic-polycytidylic acid (poly I.poly C) had a similar inhibitory effect. Rabbit interferon preparations did not cause a significant suppression in human cells. Suppression of the bacterial growth could be demonstrated in two ways: (i) by showing that treatment with either homologous interferon or poly I.poly C reduced the proportion of infected cells determined by counting the total number of cells and the number of cells with 10 or more bacteria in several microscopic fields selected at random, or (ii) by showing that a suppression in (3)H-uridine incorporation by bacteria occurs in infected cultures treated with actinomycin D after incubation with interferon or poly I.poly C. (Uridine incorporation by the bacterium is insensitive to actinomycin D.) Treatment of cells with actinomycin D before incubation with interferon prevented the development of cellular resistance to bacterial infection. Interferon preparations did not have an inhibitory effect on the extracellular growth of S. flexneri in a broth culture. These findings show that the range of activity of the interferon system apparently extends to intracellularly growing bacteria.