Four proteins unique to virus infection were synthesized in alfalfa mosaic virus-infected alfalfa mesophyll protoplasts. These proteins, P1, P2, P3, and coat protein comigrated on electrophoresis with the major in vitro translation products of RNA 1, RNA 2, RNA 3, and RNA 4, respectively. P1, P3, and coat protein were observed at 5 hr post inoculation; P2 was detected at 9 hr post inoculation. The three nonstructural proteins accumulated most rapidly early in infection until about 15 hr post inoculation; stable protein levels were maintained thereafter. Coat protein accumulated rapidly until about 20 hr after inoculation. All four virus RNA species were detected in infected protoplasts by labelling with [3H]uridine. Ultraviolet irradiation of protoplasts prior to inoculation was necessary for virus protein detection, but it severely depressed the synthesis of RNA 1 and RNA 2 relative to RNA 3 and RNA 4.