The proteins synthesized under the direction of alfalfa mosaic virus RNAS in tobacco leaves have been examined under conditions of suppressed host protein synthesis. Besides the coat protein we could detect a 22K (K = apparent molecular weight in thousands), a 35K, and a set of 54K proteins. The 22K protein is serologically related to the coat protein. The 35K protein comigrated with the 35K protein whose synthesis is directed by RNA 3 in vitro The 54K proteins are serologically related to the 35K protein produced in vitro. Readthrough products of the 35K protein cistron into the coat protein cistron have been found previously in wheat germ extracts programmed with RNA 3. Two of these proteins comigrate with the 54K proteins synthesized in vivo. Since the 35K and the coat protein cistrons are read in different reading frames the formation of readthrough products is puzzling. In viruses with a tripartite genome the subgenomic mRNA for coat protein, RNA 4, is not known to be replicated as a separate genome entity. This might indicate that proteins synthesized by readthrough into the coat protein cistron play an essential role during replication, especially in the earliest phases.