By means of carbohydrate labelling two virus-specific glycoproteins were revealed in BHK21 cells infected with Semliki Forest virus: a non-structural protein (NSP68) and a structural protein (VP53). In addition, a third virus-induced, carbohydrate-containing compound (G20) was found, which is probably a glycopeptide. In the presence of canavanine, under conditions in which viral RNA synthesis continued but no virus was produced, the glycosylation of both VP53 and NSP68 was reduced, whereas the formation of G20 was totally inhibited. The possibility that NSP68 is a precursor for VP53 is suggested by the results of pulse-chase experiments: During the chase period the radioactivity of VP53 increased, while that of NSP68 decreased correspondingly. With canavanine present, the amount of radioactivity in NSP68 and VP53 remained unchanged during the period of observation, strongly supporting the above hypothesis.