The histidine analog l-histidinol inhibits protein synthesis in both uninfected and reovirus-infected L-929 cells with virus-directed protein synthesis persisting at concentrations of the drug which completely arrest host-specified synthesis. The basis for this difference in sensitivity was investigated by comparing the response of host- and reovirus-specific polyribosomes to treatment with l-histidinol. Polyribosomes of uninfected cells are disaggregated extensively by l-histidinol treatment. Similar treatment of reovirus-infected cells causes an accumulation of virus-specific oligoribosomes. These observations suggest that l-histidinol acts predominantly at the initiation step of protein synthesis in normal cells but predominantly at the level of elongation in reovirus-infected cells. This difference in the mode of action of the drug was substantiated by demonstrating that oligoribosomes of histidinol-treated cultures accumulate more reovirus mRNA than oligoribosomes of untreated infected cultures. Reovirus mRNAs thus appear resistant to a regulatory mechanism operative at or near the initiation stage of protein synthesis in uninfected L cells.