Isolated protoplasts have been described as “naked” plant cells because the cell wall has been experimentally removed by either a mechanical or an enzymatic digestion.1 They provide a unique system for clarifying the interactions between a plant virus and its host at the cellular level. It was in 1960 that a cell wall-degrading enzyme was successfully used by Cocking2 to isolate protoplasts from tomato. The crucial advance in the field came in 1968 when Takebe et al.3 were able to solve the fundamental difficulties in preparing protoplasts from leaf mesophyll and gave strong evidence for substantial tobacco mosaic virus (TMV) replication resulting from infection with TMV 3 - 5 and TMV-RNA.6 Subsequent to 314infection by TMV, a rod-shaped virus, mesophyll protoplasts have been successfully infected with spherical viruses, such as cucumber mosaic virus (CMV), 7 filamentous viruses such as potato virus X (PVX), 8 multicomponent viruses such as brome mosaic virus (BMV), 9, 10 and rhadoviruses such as sonchus yellow net virus (SYNV). 11 © 1995 by CRC Press, Inc.