Inhibition by 3-Methylcholanthrene of Interferon Formation in Rat-Embryo Cells Infected With Sindbis Virus2

3-Methylcholanthrene (3-MCA), at concentrations of either 1 or 0.1 μg per ml in rat-embryo cell cultures, did not affect cell growth. When complete monolayers were obtained, the cultures were inoculated with Sindbis virus. The yield of infectious virus in the carcinogen-treated cultures was in general somewhat higher than that of control cultures. Interferon yields, however, were markedly decreased, the effect being most pronounced in cultures containing 1 μg per ml of the carcinogen. The same concentrations of 3-MCA also did not influence the viral-inhibitory activity of preformed interferon. This indicated that the carcinogen inhibited the production but not the activity of interferon. Sindbis virus plaques in rat-embryo cell cultures were significantly larger if the cultures had previously been exposed for 24 hours to 1 and 0.1 μg per ml of 3-MCA. This result provided additional, though indirect, evidence for an inhibition of interferon synthesis.