Johnson, Terry C. (University of Minnesota, Minneapolis), and Leroy C. McLaren. Plaque development and induction of interferon synthesis by RMC poliovirus. J. Bacteriol. 90:565–570. 1965.—Plaque development by RMC poliovirus on human amnion cell monolayers was investigated with regard to autointerference and to the effect of acid-agar overlay on plaquing efficiency. The virus was inhibited by acid-agar overlay, thereby exhibiting the d− marker typical of attenuated poliovirus strains. In addition, a lack of RMC poliovirus plaque development on HeLa cell monolayers was shown to be the result of an agar inhibitor which could be removed by NaCl extraction. By use of a simplified plaque reduction assay, it was shown that interferon production was responsible for the autointerference phenomenon. Interferon synthesis did not correlate with the ages in vitro of human amnion cell cultures. Fibroblasts originating from the chorionic membrane produced negligible amounts of the inhibitor. Interferon synthesis by human amnion cells infected with RMC poliovirus was inhibited by actinomycin D. The addition of guanidine hydrochloride to infected cultures immediately after RMC poliovirus adsorption markedly inhibited interferon synthesis, although after 2 hr (postadsorption) guanidine had no effect on interferon production.