Plagemann, Peter G. W. (Western Reserve University, Cleveland, Ohio), and H. Earle Swim. Replication of mengovirus. I. Effect on synthesis of macromolecules by host cell. J. Bacteriol. 91:2317-2326. 1966.-The replication of mengovirus was studied in two strains of Novikoff (rat) hepatoma cells propagated in vitro. The replicative cycle in both strains required 6.5 to 7 hr. Infection resulted in a marked depression of ribonucleic acid (RNA) and protein synthesis by strain N1S1-63. Inhibition of RNA synthesis was reflected by a decrease in the deoxyribonucleic acid (DNA)-dependent RNA polymerase activity of isolated nuclei. Mengovirus had no effect on either protein or RNA synthesis or on the DNA-dependent RNA polymerase activity of a second strain, N1S1-67. The time course of viral-induced synthesis of RNA by cells was studied in cells treated with actinomycin D. It was first detectable between 2.5 and 3 hr after infection and continued until 6.5 to 7 hr. The formation of mature virus was estimated biochemically by measuring the amount of RNA synthesized as a result of viral infection which was resistant to degradation by ribonuclease in the presence of deoxycholate. Approximately 70% of the deoxycholate-ribonuclease-resistant RNA was located in mature virus, and the remainder was double-stranded. The formation of mature virus began about 45 min after viral-directed (actinomycin-resistant) synthesis of RNA was detectable in the cell, and only about 18 to 20% of the total RNA synthesized was incorporated into virus. Release of virus from cells began about 1 hr after maturation was first detectable. Release of virus from cells was accompanied by a loss of a large proportion of their cytoplasmic RNA and protein.